There's a balance to strive for between making a statement concisely and making it nitpick-proof. I could have been more factually correct by stating that it's illegal to sell booze directly to minors, but that would've made the tone of my comment less rabidly libertarian.
The point, of course, is that parents can make a meaningful decision about whether they'll give booze to their kids a lot more easily than they can do the same for exposing their kids to advertisement.
Plus, I'm pretty sure that, even in those states, you could get in some sort of legal trouble for giving whiskey to a 2-year old. Obviously, we can't claim similar level of damage when it comes to advertising, but we don't seem to be willing to look more closely into it, either.