Your impressions are justifiable and analogies are cute but where does that get you?
You come across to me as a bit closed minded though it might be because you've been so riled about this experience. Did you come here to just vent or to receive constructive feedback?
I remain unconvinced there was not something else going on that you were unaware of, or could not have been aware of. I mean, what about the other MPA's and the Boss lady, where were they at during all this? I had a similar experience as you (at another SPA - and I bet others have same experience, thoughts, worries) where my MPA leaves the room, I hear moaning in next room and I'm thinking my MPA is working two rooms, but then my MPA comes back in and the moans in the other room continue (to my relief). And assuming you are here for suggestions, my advice to you is,,,,
Never give a single penny to anyone that you don't know their name, asking someone's name is not only security for you, it means the MPA is highly unlikely going to attempt to pull a fast one (you have a name to report to boss), and as well, it's just simple respect and politeness in caring to know someone by their name. Also, I make a point of asking if she is "my" MPA. Because if I don't ask then after I pay, and she leaves, the MPA returning is big bertha.
If you had a name, had reported the incident to the Boss, and still we're totally unsatisfied then it would be far less likely this was not just some big misunderstanding. I'm not saying I don't believe what you think happened, or that it might happen again. Rather, I think labelling the place a "shit hole" for what might have been one cockeyed MPA's actions is not putting things in the proper perspective (being riled up can do that to anyone) and it makes one wonder what else might have been misconstrued.