Around here (Western Washington) the relationship between yards and contractors is usually "you scratch my back, I scratch yours". In other words, yards like contractors, and vice versa, because they have the potential to bring in extra work
for which the contractor is not qualified.
Relationships are not always smooth though.
My wife works for a yard who banned a local boat repair contractor that worked out of his van. The reason? He would attempt to undercut the yard by pitching himself to other boats already in the yard. Pitching to customers like "I can fix that, and I have a lower hourly rate." A few times he said it in front of a yard employee! Not exactly the smartest business move on his part.
Hence, the yard owner banned him.
Never heard of anything legal
or illegal regarding contractors. And, I have never heard a yard "charging extra" for bringing in contractors.
But then again, no yard is short on business in these parts