i agree, i don't think there should be a law( i don't think there is, but i could be wrong)
bare feet means you can feel the pedals better and your feet cannot slip off.
if you find it much easier, just keep a pair of shoes under the seat to slip on.
I dont think there is a law on this but surely it is better to wear 'sensible' shoes rather than none at all or 4-inch heels. In an accident, couldn't either of these constitute not having control of the car