I love horror and violence, I love Battle Royale. It's about a Japanese class forced to go to some remote island to kill one another due to some legislation - "Battle Royale Act" in an attempt by adults to control teenagers. It's violent and so out of this world. Again, I know I won't be hacking anyone off with a battle axe anytime soon, or be placed in a remote island and be forced to kill one another or risk getting my head blown off by some remote tag clipped on by the Japanese military.
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Screenshot from Battle Royale - Introduction video Girl into BR
Again, I know what I can stand, honestly, I can withstand anything, but equally I know some can't - not to sound big-headed, but they are weaker than I am. There was a case in Japan referred to as "Sasebo Slashing", where the 11 yo girl supposedly have read Battle Royale manga and was obsessed with something called, Red Room. Somehow believed it was ok to slash her 12 yo classmate to death. What da heck went wrong? The product is out there, it's up to the buyer if they want it or not. If the 11yo girl was fine, sure let her read it. What harm could it cause? In this case, it contributed somehow to the death of 12 yo Satomi Mitarai.
If people are all perfectly normal and sane, what you suggest would be ideal, put everything on the shelves and let the buyer to decide, if you're interested fine, not interested, fine too. However, the world is not like that, you got the sane and the insane. Sometimes there are people that are mentality unstable fixated in one or more specific areas of questionable obsession, torture, paedophilia, gummy worms, whatever; they don't go, "Gee whiz, I know I'm a bit loony, guess I better step away from this section cos I know I will go a bit nutty if I buy this DVD". They don't do that, they don't necessary know they have a mental problem. They will rationalise it with, "They may not be interested in this film, but I sure am!" like normal people
So you got the people that go, "Bloody gory violence - right up my alley, let's buy it" then spend the entire 2hrs drooling, awing, and capturing said images of blood spewing everywhere with limbs hacked off and what not. There's still a chance crazy minute extreme guy could walk away, and most normal people do. Then there's that minute group that will keep going down this slippery path - going, "Hmm... I wonder what it will be like to grab a girl and do this to her, or do that", then that thought becomes obsessive, they become curious then feel they need to do it, to find out what it feels like. They know it's wrong, but then they internalise it and then something pushes them over the rational thought into irrational and then it happens.
Next day, you hear a story about a girl that was walking home from school, dead and mutilated.
I admire your faith in humanity with hopes that people aren't crazy, that everybody could walk away from some certain types of films or view it and it won't affect them. Sadly, there are a minority of individuals that can't do that. Something is going on in their heads, some imbalance and it has made them different to society's interpretation of "normal" people. Therefore to protect them and protect the majority of the population, there has to be some restrictions placed on some extreme end of films. It only takes a small group of people to ruin everything for everyone else. (No we didn't ruin SGU for everybody, it was SGU that ruined Stargate for the majority of SG fans and viewers).