What makes you believe that it's not otherwise legal to cover your face? Which countries are you thinking of, specifically?

Legislating against nudity is generally motivated by some sort of assumption that too much nudity risks harming some of those who are forced to view it, eg. children, sexual harassment victims, nuns etc. It is not motivated by a desire to protect people from themselves. To some extent it's also justified by long-held outmoded religious taboos against indecency.