why do you need to wonder, it's fairly obvious at least to me. it's basic human nature to be less forgiving of outsiders than one's own. this isn't exclusive to Thailand nor is it solely at a national level. in some places they don't even like people from the next street over let alone a foreign country.
also, imho, farang are a nice soft target because we might have money but we have no real power. if it was a rich Merc-driving Thai that lifted the Hong Thong there's no way this dude would report it or that Thai Rath would put his picture up on the internet.