Yes, it is legal for them to ask you to pre - approve with their designated loan agent.
But the law says that they CAN NOT require you to use him.
The way I handle it is to have my loan agent cross qualify with their loan agent. This way you don't have to have your credit run each time you make an offer.
Also it is easier because loan agents talking directly to one another speak the same language.
Andy, I hope this helps you.