Well I did have a big response planned, but I accidently lost it so, he's the gist of it.
How is this bad? All he's doing is making it so if you're under 17 you can't buy a 17 without your parent there. This means that the whole "publishers are selling mature games to kids!!111!" wouldn't have an argument any more. From now on.... responsibility would rest with the parents, and parents alone. This way, if they start bitching, we can just turn around and say "Well you bought it for him, it's your fault."
I know the US have a thing where they don't like selling AO things in stores, but this isn't AO right? It's just mature? So all should be fine.
However! I will say that if they are going to do this, it should be for films as well, and not just single out games, that's the bad bit.
If anything, this is waaaay better than the rest of them, he's not trying to ban, censor or invent new ratings or anything, he just doesn't want under age kids buying stuff. That's how it is here in the UK, and we still have everything you have on sale, and we generally have less ruckus about publishers pushing smack and conditioning to rise up on rampages on our kids.
Obviously feel free to point out any mistakes I have, I know things are a bit wonky in regards to law in the US, due to all the different states. A blanket rule may not be quite as easy as it is here in the UK.