This is not a Buddhist thing - this is a Southeast Asian thing. The whole reason behind it is going to the toilets. The squat toilets are traditional in Asia, they didn't always have real toilets, so they had to excrete and urinate in the woods somewhere. People where trained to use their left hand when wiping themselves. They didn't have toilet paper, they used water if they could, but they always used their left hand. That is why when you are giving something to someone in Asia you hand it to them with your right hand. Now it is outdated by a lot of people because in Thailand, as probably most people use Western toilets, and almost everybody uses toilet paper now. But still that tradition is an Asian tradition where you would never hand something to someone with your left hand, because that is the hand you wiped your bum with. So it makes sense.