I think it's a win-win for SPs; I am sure spas or agencies are taking money out of SPs's pockets, and it could be more than it should be.
But, in exchange, SPs do not need to worry about how she will be getting clients, how she gonna manage accommodation, not the actual security but a sense of security she will get, etc.
It's like a job: you make yourself available, show up, do your work, get paid, and go home.
Now, I know it is not that simple, and I am not talking about every spa or agency; there would be some shady spas and agencies that could be behaving like absolute pimps, but if you talk about well-reputed spas or agencies, they could be a good step to enter this industry for SPs.