I'm sure some barns do use it to maximize lesson revenue... I don't necessarily blame them because they are a BUSINESS trying to make money. :o) Although, I don't do this because my barn isn't a "show" barn and I allow insured outside instructors as well. I tell them that if one of these other instructors (that are experienced) says you can jump this horse horse with out supervision then I'll certainly take that into consideration.
A lot of barns DO blame it on insurance and not necessarily because the insurance company suggests such rules but because we are protecting our business. A lot of barns are sole proprietorship's which means to get sued would make pretty much everything subject to loss. When you've worked your tail off and spent every last dime on your barn and business you don't want to just give it away over a know it all boarder who 'thinks' they can jump 5ft. Even with insurance... if you get a claim or two, that sky rockets your rates or maybe even makes you un-insurable!
Why is it considered favoritism? You can either jump or you can't.