There are a myriad of published studies that show that exposure to violence (in any form, real or simulated) can lead to short term desensitization to violence. Long term effects are, as yet, not fully understood.
(Personally, I have childhood memories of watching kung fu movies with my buddies, and then we'd go out and emulate the movie we just watched, kicking and punching ourselves silly. My nephews did the same to their parents...not so fun.)
The American Military, however learned that in WWI & II, something like 35% of their soldiers froze on the battlefield when asked to pull the trigger at human targets. After they switched to firearms training on human silhouettes, that % dropped to single digits. Now they're doing some of that training on virual reality setups.
My point?
We live in a dangerous and violent world, and children need to be aware of that. However, its a lesson that doesn't need to be reinforced by hyperviolent entertainment forms.
And real world parents- even the best ones- cannot guard children against age-inappropriate material. This is where the law steps in.
Censorship does not refer to anyone limiting availability, but rather specifically to the government restriction of content (which may be ACHIEVED by limiting availability if the restriction is overbroad). So, if someone over 17 can get Grand Theft Auto at Wal-Mart, but my 14 year old cousin can't, I have no problem with that. We aren't restricting adult oriented material from adults, but from children.
And if that restriction has the reinforcement of fines and/or jail time (misdemeanor level only), I'm still fine with that.