Or, it's muscle memory. Dipper always buckles up, and so maybe small things like that just stick with the body.
A tiny action, a nevous twitch. Those sorts of things.
(If anyone's wondering the official reason, it's because of the committee that's in charge of making sure everything in the show is kid friendly and won't provide a negative influence, or as Alex Hirsch has described them, the people whose job it is to make up problems.)
Mostly it's not made up problems because theirs warning labels for a reason, right. Think about when the HP movies came out kids were jumping off of roofs with broomsticks thinking they could fly. It's more of a protection against idiots lawyering up for something the kid did. But I agree him on half of make problems job.
A tiny action, a nevous twitch. Those sorts of things.