Just curious, is this a USA thing? Theatres in Malaysia have seats numbered in order from house left to right.

Well the reason is fairly simple, it allows the purchaser to see how central they will be, with the lowest number being dead central. In addition, unless one knows the cinema well, the alternatives tend to leave a first timer unsure as to what a good ticket is, but aware of what a poor ticket is (1 will be far left or right, but what's central differs per theatre).