I don't know if it's more common in Japan but a college student inviting himself to a romantic party for highschoolers trying to get a girlfriend AND ordering an alcoholic beverage that gets a minor drunk should be treated as WAY more problematic than this.
Since I've been reading through this for the first time, I can answer this ancient question: they've established (notably during his new class intro) that karaoke is his hobby and he gets high scores. So it was properly foreshadowed.