You shouldn't put feet (bare or otherwise) on an eating/work surface like the tray table. On the wall... are feet really more disgusting than someone leaning on the wall with greasy hair and/or drooling while they sleep?

On the flip side though, why are feet so disgusting to people??? Your hands probably get into way more icky stuff than your feet do but nobody says "cover your hands!" or "don't put your hands on that!"