Admittedly, I haven't read the actual legislation, just news reports' summary of it, so I am likely misinformed, but frankly, I don't get it either.
Several justifications were presented, but none sound plausible to me. Supposedly, Concerned Citizens think that underage people are going to nick drinks from the bar (has that ever happened in the history of bars?) When I was underage, I just had a friend buy the drink for me and bring it to the table, or better yet, get a bottle from the liquor store and mix it at home.
I heard that one supporter of the bill said something to the effect of "when I go out to eat, I don't want to see people drinking". I think these people should stick to fast food.
There is also the classic "if kids see people drinking, they'll want to drink," which I'd respond to with "so fucking what?"
I don't think the legislation applies to bars, as it is theoretically all people of legal drinking age there anyways. I also think that existing restaraunts all get grandfathered in, so they don't have to renovate to add a mixing back room.
At any rate, it's Utah, and yes, there is definitely a weird culture here. I still like it here, but I wouldn't mind a few changes either.