No it's not, and the Game Informer article couldn't have been very informative as you can pretty much sum this issue up in a paragraph.
It's not illegal for a 5 year old to buy an R rated movie either. The entertainment industry is self regulated.
Same with video games, at a higher rate then any other form of entertainment as well. It all boils down to freedoms in the US in order to restrict the sales by law they have to classify games as obscene and not deserving of First Amendment protections. Once that door is opened they are free to do whatever else they want.
*Edit* I forgot the obligatory remark: Use the search bar, they even just improved it. Had you done so you would have found a large number of articles and threads dealing with this exact topic.