You've paid for their service, yes, but you're also asking them to give up an opportunity to market their work and attract future clients.
How did you decide they were the best to handle the wedding if you didn't see what they could do?
Lmaooo same with photographers. With the stupid condition that they must post you or you pay a ridiculous amount for “exclusive rights.” The wedding industry in Nigeria has become so exploitative and I blame Nigerians.