A) It's no surprise.
Millions of copies of games are sold every year to underage kids. It's because the stores don't always check id, even if they say they always do.
It's not Take 2's fault.
All Take 2 is really responsible for (and not even 100% responsible for) is making sure the rating is on the packaging. Some of this is also MS's responsibility, though, as well.
C) Responsible parents take other measures.
As someone already pointed out, responsible parents will have already locked their child's console from playing games with a certain rating.
D) Get a life.
Need I say more? Nope...