I beg to differ. There was a humungous lawsuit some 10-12 years ago in which the National Society of Professional Engineers (NSPE) ended up in a consent decree that outlawed their restriction on bidding for engineering services. It was considered "restraint of trade" by the U.S. Justice Department. It is now OK to bid for engineering services, of course the question you need to ask is, do you want your project designed by the low bidder?
The next question is, how do you determine someone's qualifications?