Actually, it's not that unusual, especially at professional levels. E.G. I believe NASCAR drivers sign contracts to drive only NASCAR sanctioned events.
American football players could not play for both the AFL & NFL until after they merged.
In hockey you played for either the NHL or the WHA but not both. In fact, in 1972 our best forward, Bobby Hull was not allowed to play against Russia in the Super-Series because he'd switched to the WHA.
Again, it's about the money. If I'm investing a ton of money developing a league (including player development) I don't want another group coming in and taking advantage of my investment and reaping the benefits possibly destroying my efforts. You end up with "us or them" sanctions.
Let's face it. There's a ton of money to be made in Ballroom these days and I'm not talking just about teaching. You've got products, endorsements, etc.
Unfortunately, those paying for this are the ones being taken advantage of.
The only solution will come when both groups merge. It's the Lincoln "A house divided against itself cannot survive" thing.
OB