Reminds me of some folks in school who were horrified that others were sitting on tables/desks. The idea that you'd put your butt where someone would eat seen as highly disrespectful.
Adding to this, stepping on something can be interpreted as similar. In a guide book for Thailand, it was advised that if a bank note falls on the ground, you should not step on it to stop it blowing away. Placing your foot on a picture of the king would be a sign of disrespect. You could get arrested!
Similarly, when sitting on the ground, say in a groupz you should ensure your feet aren't pointing at anyone else.
It is common in the middle east, with some leeway. Like if someone is sitting diagonal to you such that your foot isn't directly facing them, it's okay. Unless its a professional meeting, then having your foot up is just disrespectful, but I imagine that goes for most countries.