you're doin' it wrong

Actually I've seen signs from western-style bathrooms in Asia telling people not to do this, so it must have happened with some frequency to warrant the sign.

It does, but I'm pretty sure any such signs are put up because you can slip; to state the obvious.

Besides that, it makes the toilet seat dirty if you're wearing shoes, thus making subsequent users of the toilet even more likely to squat on top of it like this. Sort of a vicious cycle.