Although the pigs do not belive what Moses says, why do they allow him to return and provide him with food?

In my opinion, the pigs offer Moses food (and beer) just in case he has any news to share on the farm. Moses travels, no telling what he might hear along the way.

Another consolation around this time is the reappearance of Moses and his tales of SugarCandy Mountain. Many of the animals like to believe that they will go to a better place after their deaths, and the pigs now seem to tolerate Moses, giving him an allowance of beer every day.