I think the expectation is that since they bought the rabbit there and the rabbit was happily living there, when they return it, they expect it to still live. They think they are returning the rabbit to its "natural" environment and will happily live on the farm.

So there is no legal duty or obligation, but morally, the expectation on both parties are definitely not the same. Otherwise they would tell them that they would cook them when they return them. The fact they are hiding this information shows that expectations are not aligned.