Replying to cops... what the hell

so last night was our spring semi dance... and after the dance i was at a party.. i had all of 2 beers, then walked my gf home around 3.. there was another huge party near her house.. so there were a few cop cars around.. one of them pulls up beside us and asks me a bunch of questions... when he asks me where we're coming from i asked why he wanted to know... i didn't want him going and busting my buddy's party.. he took that as attitude, and gave me a 130 dollar ticket for underage drinking, because i had been kind enough to tell him that i had 2 beers at the party..

what the hell is with that..