Not being able to independently either verify or disprove your info, I will give you my own personal (like this means more then two ounces of rat urine to somebody I am sure) benefit of the doubt and believe without my own personal knowledge about that.
It would make sense to be illegal, as even though you did pull the wrong trailer back to where you were going, you did accomplish a couple of things...you brought their equipment back, you possibly brought one correct trailer back, and the most important item is...you were still covered under their insurance and workmens comp in the event of an accident during your trip back...
Penalizing someone with refusal to pay, or however you want to look at it would seem to be illegal, and seemingly a breach of contract per se....when the cake maker puts the wrong color in the second layer of frosting, does he/she not get paid for the entire time it took to make the cake? Of course they do. There may be a personnel issue with some discipline involved, and termination if repeat of the problem happens in the future, but that is to be expected.
You know thinking about it all...I dont think I have ever heard of anyone getting pay withheld because of this...
Has anyone?