Part of the problem with your question is that you’re not adequately considering the other differences that exist in the areas you’re addressing.
For example, how many warning signs in general do you see in the parts of the world where squat toilets are generally found? Not many, I’ll wager. There are no warning signs to “wash with your left hand”, either. It’s assumed that the audience of people using these facilities have all grown up there and don’t need a lot of daily reminders. They aren’t tourist areas for Westerners to visit, and there aren’t a lot of Western refugees and immigrants (not enough, anyway) to make warning signs worthwhile.
Aside from that, as @Lawn points out, anyone who could successfully sit on a squat toilet would (probably) not soil it unduly for the next user, as someone standing on a Western toilet seat obviously would (and does). So a warning sign there would serve no purpose other than to educate the completely clueless.
And aside from all of that, it’s simply a lot easier to squat on a toilet seat than it is to lower oneself to the floor to sit on a squat toilet. Someone who is actually going to attempt that is past the ability to educate with a sign, I think.
So someone coming from a place where squatting is the norm, and who has no previous experience with sitting toilets, might simply assume that Westerners have just elevated their toilets for reasons unknown, and provided hinged rings to stand on, also for unknown reasons, and just “do what they always have done”. (And there are a lot more of those people in the sign-crazy West than there are the reverse.)