Old Major has given the animals a set of rules or commandments that define the theory known as Animalism which is at the heart of how the animals are expected to behave. Old Major is very specific about what they should and should not do, so in Chapter 6, when Napoleon announces that he is going to start trading with the humans, this behavior goes against Old Major's directives, "whatever goes on two legs is an enemy."
"I have little more to say. I merely repeat, remember always your duty of enmity towards Man and all his ways. Whatever goes upon two legs is an enemy. Whatever goes upon four legs, or has wings, is a friend. And remember also that in fighting against Man, we must not come to resemble him." (Orwell)
The animals learn that the pigs have started sleeping in beds, in violation of Old Major's instruction to not become like man. His speech is very specific about beds, tobacco, alcohol
"Even when you have conquered him, do not adopt his vices. No animal must ever live in a house, or sleep in a bed, or wear clothes, or drink alcohol, or smoke tobacco, or touch money, or engage in trade. All the habits of Man are evil. And, above all, no animal must ever tyrannise over his own kind. Weak or strong, clever or simple, we are all brothers. No animal must ever kill any other animal. All animals are equal." (Orwell)
Old Major's warning in the beginning of the book is actually prophetic, once the pigs start behaving like humans, they become more abusive and tyrannical than the farmer. Old Major's vision of a world where all animals could be equal included a specific instruction on the vices of man, as a way to protect the animals from the worst kind of creature, according to Old Major, humans, little did he know, that the worst kind of creature, worse then a human, is a pig.
He could not have known at the time that he was actually warning the animals against the horrors that awaited them as a result of being "equal," but then that commandment is changed as well, "all animals are equal, but some are more equal than others," is the revised version.
And the pigs prove this to be true, while the rest of the animals serve their needs that far exceed the demands that Farmer Jones ever made on them.