Does game censorship protect kids?

In stores like Game-stop are underaged minors able to easily get their hands on "Mature" games? Do you find game vendors are good at protecting kids from objectionable content and do they card on a regular basis? In my personal experience Game stores have been very good at this. But that may not be the case everywhere. Also why do some parents get upset when their son or daughter plays an offensive game yet they were the ones that bought it to begin with? Does game research help prevent these accidents? Should parents be more active in learning about a game before buying it for their children? If they know a game is "mature" and get it for their kids are they not the ones at fault instead of the vendor?