I don't actually know, but I suspect it's more along the lines of what bravecows said: people just think it's dirty to sit somewhere where everyone else had, particularly when people accidentally pee on the seat. I, uh, think whoever told you it was a cultural/religious thing definitely misunderstood! At least for most of Asia.

... one of my (white) coworkers actually said that about Indian people as well and I rolled my eyes at him and was like, "Dude, if you have been near toilets that aren't cleaned that often, you would want to squat over them too."