A fairly large amount of it is, I think, not wanting the kid to get in trouble later on. Even if you're OK with it, the society they live in may not be, and as

A fairly large amount of it is, I think, not wanting the kid to get in trouble later on. Even if you're OK with it, the society they live in may not be, and as a parent you're preparing them to live in that society. If you don't, they get punished and alienated. Which is worse, temporarily supporting nonsense, or temporarily causing problems?