Therein lies part of the problem. A large number of those places that disguise themselves as legitimate clinics are actually covers for human trafficking.

Depending on where you are, becoming a massage therapist can take a couple years and cost thousands of dollars, require exams, and licensing. Legitimate massage therapists aren't going to risk throwing everything away for a couple extra bucks for a handy. If sex work was legal, people wouldn't have to masquerade as much behind legitimate business.

As for massage being snake oil, I guess it depends on what the person is claiming it's going to do. Like I said, I've been doing this almost 20 years now, and have worked in health clubs (mostly doing sports/injury recovery/rehab stuff), education (teaching massage, anatomy, physiology, pathology), and now working hospice the last several years. Massage can be great for pain relief, relaxation, and general comfort. For some people with circulatory issues like lymphedema, massage can help reduce the fluid settling in their extremities. Unfortunately historically there's a lot of questionable and downright bullshit practices that have been attached to massage, but massage on it's own? If you like to be touched, it can be a good thing.