In what culture is the "thumbs up" sign insulting?

My retired (working PT as a tutor) mother, my nephew, and I were going out to lunch last week, and as part of her training to work at a “learning center,” was exposed to numerous “sensitivities” to be avoided. One of which was a thumbs up.

Why and where is this offensive? All East Asians and Indian folks I’ve known never seemed to shy away from it. The numerous East Africans I’ve worked with – I don’t know, but I never found it a problem. MENA cats seemed cool with it too, flashing the thumbs up when “mission accomplished.”

True? Not true? Details I’m missing? Cites/links?

Fixed link.