> Because some parents would.
Some parents complain about sex ed and vaccination, satisfying the lowest common denominator doesn't really work.
If some kid showed actually NSF-School images, such as nudity, to other kids and it was a first time offense a warning should suffice. If it's a repeated offense then maybe the kid needs psychological help.
Just as a hypothetical scenario, there's the possibility that a kid shows others a picture of for example Michelangelo's David (or similar art piece), do you think that kid should be punished for showing nudity to other kids?