Firstly, it's not that "Reddit is a bastion of some fairly horrible groups", but that humanity happens to contain some fairly horrible groups. Reddit is just software. The same groups would otherwise use vBulletin, Usenet, or whatever other forum software to band together. You're conveniently forgetting that the same software + website are host to /r/fitness, /r/mensrights, /r/GetMotivated, and other very positive groups.
Basically, reddit is the early-21st century's Usenet. That's all there is to it.
Why not generalize a bit further and say, "The Internet is a bastion of some fairly horrible groups"?
Thats where the question of morality enters. Vbulletin could choose not to sell their software to racist hate groups, the same way a newspaper chooses not to publish a racist screed in their editorial section, the same way a hosting company can choose not to host pro-anorexia sites.
Reddit could choose not to allow the horrible side of humanity to use their platform. Even tumblr has standards. There will always be awful parts of human nature, but through our moral choices we minimize or promote them.
Basically, reddit is the early-21st century's Usenet. That's all there is to it.
Why not generalize a bit further and say, "The Internet is a bastion of some fairly horrible groups"?