IANAL, but...
It isn't illegal for your dad to buy you beer if you are under 21.
It is illegal for him to buy your friends beer.
It isn't illegal to buy beer if you are under 21.
It is illegal to sell beer to someone under 21.
It is illegal to use a fake id.
It is illegal to buy beer for anyone under 21, even if you are under 21.
Public intoxication is also illegal in various states.
I agree with your outrage over the 'drinking age,' but you won't get thrown in jail for having a beer if you aren't 21.