This is a very interesting question to consider, because you seem to infer that Joe should have refused to release Pip and to keep him as his apprentice. This event of course occurs in Chapter 18, when Jaggers turns up and enters Pip's world with the news of his great expectations. When Jaggers asks Joe if he will cancel Pip's indentures, for "his good," note how Joe responds:
Lord forbid that I should want anything for not standing in Pip's way.
Joe clearly thinks that standing in Pip's way against Pip's own will and to prevent Pip's good future that Jaggers refers to would be very wrong indeed. In a sense, he has an argument, as Joe is such a kind and loving man that he could never assert himself so strongly as to oppose Pip's will, just as he could never assert himself to protect Pip from the ravages of Mrs. Joe.
However, let us for one moment consider the reasons why Joe should have refused this request. Firstly, Jaggers is a complete stranger, and they have no way of knowing whether he is somebody to be trusted. How does he know whether Jaggers is actually speaking the truth when he says that Pip's great expecations will be "good" for him? Secondly, Pip has been an orphan who grew up in a secluded country location all of his life. Joe is his only family now. He could have refused arguing that Pip could not be taken out of his world and placed in a very different world of which he knew nothing. Lastly, Joe could have refused because of his love for Pip and his fear of the way that great expectations will corrupt and change Pip in terms of his character for the worse.