There so many mortgage regulations on the books in this day in age, it's hard to keep track of all of them. The government is constantly changing what is and isn't allowed by lenders. The most recent set of laws handed down at the end of 2015 were all designed to protect the mortgage borrower, and are very much aimed at cracking down on cronyism.
If you've ever been to an on-site home builder, such as DR Horton, Lennar, Drees, etc., you'll notice they have an in-house lender, or affiliated company, they will usually try and mention as much as possible. While it's not a bad thing for them to do so, it does come with its own set of rules the on-site rep (who works for the builder, and not the consumer) may not be aware of. For example, it's illegal to offer incentives for the buyer to use the preferred mortgage company and not allow the same incentives if they want to use another mortgage company. It's a violation of anti-steering laws as part of the Consumer Financial Protection Bureau. While many do still get away with it, they shouldn't be, and you should be aware.
Let me make sure i'm clear about this. The BUILDER cannot offer housing incentives to use a certain mortgage lender which are not available to the buyer if they choose another lender. The LENDER themselves are allowed to offer any sort of incentives, and most do, in order to get your business. The BUILDER is not allowed to offer the house to come one way with Mortgage Company A, but a different way if using Mortgage Company B.
The preferred lender may not always be the best option for the consumer. They may have higher fees, higher interest rates, etc., where a competing lender can beat all of those things. But if the competing lender doesn't get the buyer, say, $5000 in upgrades from the builder, then the buyer may opt to go with a less favorable lender. This handcuffs the buyer into something they may end up paying more for when they shouldn't have to.
This leads me to my final point. ALWAYS HIRE A REALTOR when you buy a brand new home! I've helped several clients build new homes, and have seen many different things play out. It costs you NOTHING to use a realtor to buy a home. The realtor is there to be a safeguard against things of this nature, as well as the thousands of different things which can come up in the building process. The sales price doesn't get lowered if you're not using a realtor, which is a common thought. So why go about it alone? I've helped clients fight builders who didn't want to finish contractual obligations, who didn't meet code regulations, and so on. It pays to have your own representation.