Constitution forbids 1) interference in contract 2) bills of attainder. ATTy General determined that the ban by congress was both. the government may not create new contracts, but any contracts already in force will be honored.
Acorn makes money by contracting with the government to provide services to the disadvantaged, like how to evade immigration laws, how to sell underage girls for prostitution, that kind of thing. If Acorn had a contract prior to the ban, they will continue to be paid.