What I sure about is that no one force a person to have continious exposure to such propoganda on 4chan. It's not like it's some government funded website that everyone has obligation to visit due authorities requirements.
Being exposed to this kind of "humor" is deliberate choice. And even 4chan is not some single entity: there are plenty of boards on other topics that nowhere as extreme and sometimes they better moderated than e.g reddit , facebook or linkedin.
That kind of discourse is addictive and even though you have a choice, we as a society don’t always allow kids to make their own choices — especially around addictive, societally harmful issues.
Subjected to enough propaganda, literally anyone will succumb to the propagated viewpoint with very few exceptions.