I agree totally with all of your reasons why a background check should have been done with all who work with children. However..we are talking Thialand here...not USA.. I think we tend to put USA standards and law across the board in our thinking.
My brother is currently teaching ESL in Thialand, and i believe all he had to do was take some short course in order to do so. I don't know what kind of background requirements there were. I'll certainly try to find out. I do know that he is not even a college graduate, and makes very little!!
Schools in Thailand may be careless about their hires but this job clearinghouse is in the U.S. IMO he still has a duty to screen the applicants.
But the company has a moral responsibility to protecting children and promoting quality teachers. The web site has allowed a monster to promote himself. We know that this "clearinghouse" does not even mention criminal history in its application. We do not know whether this company has allowed other monsters to be hired.