It's a distinction English makes: you pay a bill/an account, but you pay for [something you purchase]. You can find a couple more examples in http://www.mijnwoordenboek.nl/vertaal/NL/EN/betalen: u hoeft dit niet te betalen. - you don't have to pay for this ("*you don't have to pay this" would be wrong if it was referring to something purchased, but it would be correct if it was a bill).
It's really confusing. In portuguese we do not need a preposition to state that the chicken is what we are buying. We do need a preposition when we refer whom we are paying to. But colloquially this preposition is dropped and the context shows if the chicken is being bought or if he/she is the one that is receving the money.