There are many "legitimate" schools in China, and still they are crap. "Legitimate" says nothing about the quality of education students may get, and it definitely also does not say anything about the treatment of foreign teachers. And it is the latter that is of main interest here.

Absolutely correct, Turnoi. The bottom line from an expat's viewpoint is how crap is intolerably crap. That of course will vary from expat to expat. We all have our personal moral codes and levels of tolerance. But probably, desperado expats in China, of which there are many, will put up with much more crap than those of us with academic qualifications and hence other employment options elsewhere.
As far as I'm concerned, once the Chinese side tries to break my contract, or is found out trying to con me by telling me stupid oral lies, it's time to move on, of course only when my contract comes to its natural conclusion. No use leaving early and thereby cheating yourself financially.