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.
It's obligatory.
Checking something someone told you about on the smartphone not so much. (Unless you are in high school... then the reverse:)