Death threats aren't the only kind of harassment. Check out the comments on any popular Youtube video, or most news sites that have comment sections. There aren't many of those left actually, because they tend to fill up with ridiculously high levels of harassing and even threatening remarks. Twitter users had to roll their own group-blocking solution (and Twitter has built-in some of these features recently) so communities can collectively compile ban lists. There's just too much shit out there for any one person to deal with.