It may not derogatory, but sometimes I just want to be left alone. Does it really brighten their day so much to see someone with white skin and a big nose? Is it really that amazing? Can you imagine if I taught my kids, in USA to point and say "blackie! blackie! blackie!" every time they see a black person? How do you think that would go down? So Thai kids are taught never to ask things or question authority, but harassing white people is OK.