Well, this was because of THIS!
BUT OF COURSE IT WAS. Here's the story from the Baltimore Sun:
[Director John Waters and Justin Bieber] shared the sofa last week as guests on Britain's "The Graham Norton Show." Bieber was there being a heartthrob, while Waters promoted his book "Role Models."
Not long after the shrieking died down from Bieber's introduction, the 16-year-old asked if he could say something. He leaned across to Waters' end of the couch and pronounced as only a teen could: "Your 'stache is the jam."
"Thank you. Thank you very much," Waters replied, a big smile on his face. He then pulled an eyebrow pencil from his pocket and handed it down to Bieber, saying, "If you want to try it…. Can I draw it on you?"
Bieber took the pen but declined Waters' offer of artistry. "I'm good," he demurred. "Maybe later."
Sure enough, as soon as the show ended, Bieber drew on a version of the mustache.