1) Because it's socially accepted and *fun* as hell after a few dozen too many.
2) To serve as a warning that the occupant of a home is armed and will let a hot dozen off in your ass with the quickness.
Army here...but have you ever considered how much of a pain in the ass it is to do midair refueling? (Mind you, I'm Army, so I know only a FEW of the headaches) Getting the tanker there, keeping it from getting shot down, and then the actual refueling itself which is IMO more dangerous than landing a multi-ton barely guided missile onto a target that is moving on all three axes.
Interservice rivalries aside, the Marines are generally the most frugal of the branches, so I'll give them the benefit of the doubt.
Look again.