The line is where they, as a company, decide to put it. I would not go to a social networking site that had gore or porn polluting my feeds. Similarly, you might not go to a website that disallows any content. That's our prerogative. It's facile to paint it as a free speech argument.
Why would you subscribe to users that post gore or porn to your feeds?
Tumblr has plenty of porn, but you can certainly use it without ever getting it on your dashboard. It's just a matter of, you know, not following the people who post it.
