People go to the bathroom to relieve themselves, people use Yik Yak to be entertained[1] by the hurtful gossip people have to say about one another. When I go to a urinal I'm not scanning the walls for slander, and any slander that's there isn't instantly accessible anywhere on campus.

[1] Obviously some users check Yik Yak to see what's being said about them, teachers may check it to be aware of what students are being targeted, etc.