Did you hear about the millions of people that ride motorcycles safely to work and then home every day? Yeah, probably not - I didn't either, but I did hear about that horrible accident, and that made me worry about motorcycles being safe.
The fact of the matter is, of all the people using Tor, you never hear about the good things that Tor users do for sites, just like this one. You only hear about the spammers, trolls and psychopaths that use the service as a way to evade efforts to keep them from creating the disruptions that they do.
We (Stack Exchange) do not block Tor exit nodes, but a considerable number of them are currently being tracked by our spam prevention and abuse mitigation systems simply because Tor is a tool that does not discriminate against endeavor - a lot of people up to no good happen to use it because it fits their particular need quite well.
It makes me sad that the few - those that feel enabled or entitled to be complete and total miscreant jerks can result in such an inconvenience for those that simply want to not be tracked, but it's unfortunately the way that it is, at least for now.