The construction of this metaphor can be considered slightly inconsistent. Willy tells his boss, Howard, that he cannot eat an orange and then throw the peel away, which of course, he can, and most people do. But Willy is in a tortured state, and it is clear to see what he really means to say. He is saying that Howard should not treat people, namely people like Willy, as if they were oranges. Willy is not an orange, and therefore it's wrong of Howard to use him up and then discard what's left of him as if he were something be consumed.

The metaphor is a commentary on capitalism and on the abuse of workers by their employers. If Willy came to Howard as a fresh, young "orange," then we can compare Howard's treatment of him to the behavior of a man who peels the skin off the orange, eats the best parts of it—the "fruit"—and then simply gets rid of what he doesn't want. Obviously, this is what is usually done with oranges.

Perhaps Willy means that it is also what is usually done with workers: the best parts of them are consumed by greedy employers, and then the empty rinds, the leftovers, are discarded, because they are not useful. Willy's comment is that, whether this is what is commonly done or not, it shouldn't be. People should not be treated like oranges, because they are not disposable material to be consumed by the system.

What Willy Loman means by this expression is simply that it's wrong to exploit a man and then discard him like he's nothing. That's precisely how Willy feels after being fired by his boss Howard. As far as Willy's concerned, the company's had the best out of him all these years—they've eaten the orange, so to speak—and yet they're now throwing him on the scrapheap like some worthless piece of orange peel.

Willy is undoubtedly hurt by his treatment. He's given the best years of his life to the company, and now they're showing him the door. Although Howard's actions seem somewhat unfair on the face of it, the sad fact is that Willy's become a dinosaur in the world of sales. The world of salesmanship has changed dramatically, but he hasn't. He's been left behind by changes in the business and finds himself unable to compete with the current generation of hotshot salesmen, each of whom is considerably more successful than Willy.

Even so, there's no denying Willy's right to feel used after giving the company the very best he had to offer. Like many people in a capitalist economy, he's been cruelly discarded once he's served his purpose, once his "fruit" has been eaten.

This quote is from Willy Loman as spoken to Howard Wagner, the supervisor (and son of Willy's previous supervisor) at the company for which Willy works. Following Linda's advice, Willy goes to his boss to ask him for a permanent position within the office so that he does not have to travel so much. He also asked for a salary position, as Willy had been living off commission but, since he was not selling, he was really able to make it thanks to the handouts of money that he would ask his neighbor and friend Charley for.

The words are also a reaction to what Howard had told Willy after a long time trying to divert the situation.

I don't want you representing us anymore.  I've been meaning to tell you for a long time now

To which Willy reacts in dismay for several reasons. First, because Willy still abides by the old code of friendship. He believed that Howard Jr. should reserve a special place of reverence to Willy as he had known him since he was a child. Moreover, Willy still also abides by the old code of loyalty: if you are loyal to the company the company should be loyal to you and keep you as an employee. Not the case; finances were looking up and productivity is a must in the mid 1940's; Willy is a piece of the old school and he must be taken away.

Hence, when Willy says

you can't eat the orange and throw the peel away--a man is not a piece of fruit.

he reflects this old school mentality: that you cannot just use someone with whom you co-exist, work with, and share with year after year and then one good day turn around and get rid of that person as if he had never been there. It is no different than feeling like trash: you are no longer useful, so we will throw you out.

Willy did feel eaten from the inside out by the machinery of selling: he neglected his marriage because of it, he got into debt because of it, he worked his heart out because of it and now he is no longer "needed". Not only does this emasculates him in the eyes of his family, but also makes him feel quite betrayed by those for whom he feels that he had done so much for. Like a "piece of fruit" the company indeed consummed most of his life and, now that he has none that he could be proud of, they just decided to throw away his "peel" and get rid of him for good.

"A man is not a piece of fruit" is a direct rebuke to consumption. The company where Willy Loman worked as a salesman, first for Howard's father and then for Howard, is discarding him, as one would discard an orange peel. Like an orange peel, they have no use for him in his elderly, senile state. 

They have taken, or consumed the best of him—the "fruit" of his youth, as a previous educator has mentioned—and they are now throwing out what they find distasteful about him: his age and slowness. 

Indeed, the metaphor is uneven and somewhat nonsensical, but I think that Miller intended for it to be. Loman is losing his mind, which would make him unable to form good analogies. Howard has also reduced him to a feeling of ineptitude. His language demonstrates that.

Willy here is reproaching Howard, his boss. Willy has worked hard for Howard's company all his life. He has done his best for this company; they have taken his best years, plucked the 'fruit' of his youth, so to speak, but now that he's old and can't compete with the other salesmen, they're just casting him off in the way that one might discard fruit peel. It is a very telling quote; Wily feels he's had no reward whatsoever for all his years of dedicated service. Willy feels that times have changed and there is no personal warmth in the workplace any more, that it's all become too competitive and commercial. Willy recalls his friendship with Howard's father Frank but emotional appeals get him nowhere with Howard.

Willie's choice of a metaphor is a poor one. It either shows a flaw in Arthur Miller's writing or else Miller was intentionally making Willie look unintelligent and unimaginative. Eating an orange and throwing the peel away is exactly what you can do and what is generally done with oranges and orange peels. What Willie was probably trying to say is something like this:

You can't use a man up and then throw him away the way you eat an orange and throw away the peel.

Whether it was Arthur Miller's mistake or intention, the metaphor, or simile, or analogy, or whatever it is supposed to be, is almost comical in its ineptness.

This metaphor, however inept, is at the heart of Arthur Miller's thesis. Under capitalism, people are used up and discarded without any consideration for their humanity, welfare, or personal feeliings.