Controlling parents

I'm 21. My parents don't promote my drinking, but they allow me to go out. I am not allowed to drink at home or have alcohol at home. I was at my aunt's house at a family gathering I went to with my parents. I was not driving so when my uncle offered me a beer, I accepted it. When I walked up to where my parents were, my dad came up to me, SNATCHED the beer out of my hand, and said "who's drink are you holding?! Who are you taking this to?!" and handed it to someone else. Am I crazy for being frustrated? My mother said that because they don't drink or promote my drinking, not drinking in front of them is respectful and to be expected.