It's a difference in cultural expectations. As I found out recently, putting one's feet on the table is considered rude in Germany.

If you come from a culture where picking your nose in public is impolite, then it might disturb you to see someone really going at it. In the US, using the middle finger is generally considered rude or offensive, but the act on its own is meaningless and not especially prone to damage anything.

But acts do have cultural associations, which is why putting feet on a table, or window frame, may disturb others, even if it doesn't disturb you.

Personally, I have no problems with foot-on-frame.