Legal? Ethical?

So I’m looking through MLS and see this:

“Great starter home for any size family right here!!!! With a little TLC this property could be a true beauty!!! Seller requires pre-qual through Wells Fargo NO EXCEPTIONS! Contact XXX for more information @ 972-XXX-XX16. Must have pre-qual through Wells Fargo to submit offer. Fax all offers to XXX-XXX-XXXX. Incentives may apply.”

Is that legal? You can’t ask someone to go with one particular mortgage company to submit an offer, can you?