The closest parallel which I've seen is people being racially abused and members of the public rarely if ever intervene in those cases. Like most people, I've seen plenty of fights and nobody (myself included) stepped in so I think it's just fantasy to think that the oh-so-moral-whitey possesses virtues which Thais lack; it's obviously popular on forums like this for people to claim that if they were there, they'd have the fekin lot of 'em but, really, it's bullshit.