I know the answer or part of it based on my knowledge of how the business works but he should be th3 one addressing it.
Howevr there are many things fans ignore about how the boobs world was and is run.
do you see Laurine working for someone different than Ferres? Do you see Yonia wanting to work for someone differ than Daktari ? Did Luna -aside from 1 shoot last year- worked for Score after working for Daktari? Did Viola ever accepted one of the 10 offers Legal Porno sent her? Do you remember Tessa interrupted her Ziahy shoot mid shoot?
Why did some Romanian sister stopped being booked by Score after working on Nadines site?
And not there are no exclusive contracts. (Maybe Tessa had one. But I don't know for sure).
One of the answers to many of those is: once certain producers got their hands on certain "cash cows" they would do everything they can to prevent the girls working for others, as it would get their revenue down. And exclusive contract was not anything someone signed as it would-be too expensive.