4chan is light years beyond the point where people just posted "Hitler did nothing wrong" to piss off the righteous.
People acting like hateful idiots for fun eventually brought actual hateful idiots. This brought people who like to spread their ideas to actual hateful idiots.
There's no difference between idiots in churches and idiots on 4chan - both are idiots, both eat up whatever bullcrap is served to them, and both are harmful to society. And neither means that churches or 4chan are bad in themselves.
We must be careful before starting to excuse idiots for their idiotic behavior based on "bad influence". Society is a complex mechanism, and censorship inevitably brings unforseen consequences. Human mind is also complex, and anti-social behavior can sometimes be traced back to early-childhood traumas. Does that mean we should ban free parenting and raise infants in government-approved facilities?
Erroneous argument - shouting "fire" in a crowded theater has a direct, predictable consequence of people acting on it. Posting stuff on the internet does not.
People acting like hateful idiots for fun eventually brought actual hateful idiots. This brought people who like to spread their ideas to actual hateful idiots.