I've seen a few Asian providers, both independent and agency.  My ATF was an independent Asian American who definitely knew what she was doing and was in no way coerced into it.  Agency providers are for the most part here to generate an income over the course of their visas before they return home.  Are they coerced?  I don't know for sure one way or another, but my guess is that reputable agencies that advertise on well known websites are not the ones you need to worry about.  I've seen maybe half a dozen agency providers over the past few years, and never once did I pick up that they were being coerced.  A couple seemed kind of bored or at least mechanical.  Let's face it, you don't have to be an Asian agency girl to exhibit that sort of behavior.  A few have been incredibly friendly (despite the language barrier) and totally into what they were doing.  You can read my reviews for the details.  I do wonder about some things though.  Firstly, I think that by industry standards, Asian agency providers are "high volume".  That must translate eventually I would think, into becoming burnt out, jaded, and mechanical.  Secondly, as "high volume" providers, there must be an increased likelihood of developing sexual health issues.  Thirdly, as visitors to this country, I don't know what access they have to health care, regular testing, and other services.

If your concern is whether providers are being "trafficked" or forced to work against their will, you're far more likely to find that with some black street walker working for a gorilla pimp than with an Asian provider, for a number of reasons.  

For starters, the definition of "human trafficking" is rather ill-defined for a number of reason, and left that way by people with agenda to peddle. Yes, "human trafficking" CAN mean sexual slavery or coercion, but it usually refers to illegal immigration, so to what degree are we talking here? People being physically smuggled across our border? Use of phony documents upon entry? Overstaying student/tourist visas? Phony arranged marriages? All of those are of course immigration law violations, but in > 99% of those cases, the person coming in illegally participates voluntarily, so it's not exactly coercion.

Secondly, just as illegal workers sneak across our border with Mexico for economic reasons, most of these Asian women come here for similar reasons. Fact of the matter is that in most Asian nations, if you're a 30+ year old unmarried woman, you're considered an "old maid" and no longer desirable. Add lack of formal education and/or job skills, and your prospects are pretty miserable. OTOH, a 30-something Asian woman is not only still quite attractive in the eyes of most American males, but even considered a bit of a prize, so she has a marketable commodity to offer. In addition, many Asian females consider American men to be much kinder and considerate (and less sexist) than their Asian counterparts, and the reputation of American men as being "white knights" due to their willingness to overlook the past of these women (more than one "trophy wife" of some highly paid executive met her husband in a hostess bar or massage joint, either her or over there). There is a strong incentive for these woman to come to the States and take advantage of that situation, whether legally or otherwise.
Some of them work as mercenaries, saving up money so they can return home someday. start a business, and live a life of relative luxury in a place where they are familiar with the culture and the cost of living is low. Others decide they like it in America and choose to stay here permanently.

Thirdly, observation of most Asian providers makes it clear that they aren't "sex slaves" or coerced, especially when they have money in their pockets and are free to come and go at will. One of the major complaints of Chinese and Korean massage establishments is that their girls often start working other establishments, even traveling out of town on weekends to places where they can make more money quickly. It's hard to believe your ATF massage girl is being "trafficked" when she has a handbag and shoe collection that would make your SO green with envy and drives her BMW down from LA for her weekend appearance at your local massage parlor.

Last but not least, if some woman IS being trafficked, you're not going to find her in an establishment catering to Americans. As I mentioned. American Men have strong WK tendencies and a high risk to such an environment. Any sense that a favorite provider is being worked against her will brings out the Captain Save-A-Ho in the vast majority of American guys. I don't doubt that this may indeed be the case for a very small number of females, but any such women are going to be held in a location where the only customers are people of the same ethnic background or country of origin.

I can go on as to the reasons why the whole "human trafficking" thing is being overblown, but I will save that for another post..