They consider it a form of stealing their spirit--it was the same in Africa except that for a dollar, they'd suspend that belief. Although it is generally best to obtain permission before taking someone's photo, sometimes the temptation to just do it anyway overwhelms us. For the respectful traveler, it's an uncomfortable decision to just grab those action shots that we'd kick ourselves later for missing. For those cases, use the best camera available that has a tilt viewfinder so you don't look so obvious. Afterwards, to clear your conscience, make a few donations to a few locals in obvious need.