The animals have become accustomed to the idea that they can exist independently from humans. They like the idea of being self-sufficient, and that was Old Major’s dream. They don’t trust humans, and a wary of being under their control.
Why then do we continue in this miserable condition? Because nearly the whole of the produce of our labour is stolen from us by human beings. There, comrades, is the answer to all our problems. It is summed up in a single word--Man. Man is the only real enemy we have. Remove Man from the scene, and the root cause of hunger and overwork is abolished for ever. (chapter 1)
If the animals are trading with humans, they have not removed them from their lives. They could still take advantage of the animals.
Once again the animals were conscious of a vague uneasiness. Never to havev any dealings with human beings, never to engage in trade, never to make use of money--had not these been among the earliest resolutions passed at that first triumphant Meeting after Jones was expelled? All the animals remembered passing such resolutions: or at least they thought that they remembered it. (chapter 6)
As it turns out, the animals were right to be nervous. The trade with humans for things like alcohol only benefitted the pigs, and made them more like humans.