All of these people are travelers, or like to think they are. All of them like to think they're worldly and experienced, that travel has imbued them with perspective broader than one gets while sitting on one's ass at home, never leaving one's city. They like to think they're part of a global community.
That means, to them, disrespecting those cultures they visit and doing whatever the hell they wanna. In Phnom Penh I happened to find a very useful tourist map for fun things to do in the city, and right there was printed a list of things to be mindful of so as not to offend the locals, foremost being to "keep your upper arms and thighs covered." Simple rule, basic rule, and a rule not exclusive to Cambodia. Yet somehow each of the above (and the hundreds they represent) seemed to miss this in any guidebook or posted sign at the airport or passport station. Or, worse, they did see the notice and willfully chose to ignore it.
Note: Please to note the elderly man second from top, second from left. I first saw him strutting through my hotel's lobby with a prostitute, and I figured he was shirtless out of convenience. When I saw him the next day, also shirtless, wearing the same dress pants, coming back from the morning market and without a prostitute, it was harder to tell whether he believes Cambodia is the place to show off his physique or he's simply gone senile and needs some real help.