Forgetting the visual grossness of bare feet in odd positions for one moment, I'm not sure which would be worse, someone putting the feet on a tray table (as pictured in one of the links), or given the hygiene practices of some, putting their hands on a tray table. Most people don't use their feet to wipe themselves with toilet paper after a "number 2" visit to the toilet, but do with their hands. And studies have shown people don't always wash their hands after visiting the toilet. So I guess it depends on where said feet have trodden when out of their shoes.