Nope. I'm not. The spa owners may be "providers". The girls themselves are mostly not.
See https://www.pri.org/stories/2018-01-25/across-us-many-illici...
for one example. Another: http://www.columbusmonthly.com/article/20150507/LIFESTYLE/30... Or this: https://www.texasmonthly.com/articles/the-lost-girls/
You'll find similar stories for every major US city. They are slaves lured here under false pretenses and/or sold to traffickers by their families. Generally they are forced to stay in the "spa" 24/7 paying off false debts that will never be paid off.
I did mention this is mostly unrelated to Backpage.
Edit: Downvote at will, but provide some evidence that the women working in Asian spas are there by choice.
My whole rant was directed at GP's claim of " it is also apparently quite difficult to find actual trafficking victims". It isn't difficult. It's pretty clear that most of those places are benefiting from traffickers, thought I don't know what percentage of overall "sex for sale" they represent.
That doesn't mean all (or even most) prostitution is related to trafficking. However, I stand by my claim that most Asian spas and massage parlors are mostly supported by trafficking. It isn't difficult to see, find, etc, in any major US city.