If you think that I as a host should not have the right to throw abusive visitors out of a gathering at my place, you're a fucking idiot.
When you've advertised your party as a "come one, come all, visit the tea room for the lace doilies, or the BDSM dungeon, or jump into the political mosh pit!", then yeah, it takes a lot of damned gall to draw lines around one particular niche puritanical issue after everyone shows up to the party. "Whoah now! Sure, we said we'd have beer for everyone, but we didn't mean beer beer, we meant O'Doul's!"
That said, you technically have it right - The owners of Reddit have every right to decide what content to allow on their site. Decisions like this have consequences, however, and we've seen it on site after site after site - Get too popular, start banning "offensive" content to appease the advertisers, and watch your userbase move on to the next "Wild West" site.
If they really want to ban something to promote harmony among the users, they could just get rid of TwoX - But of course, that would look bad, so instead, they will slowly ban everything incompatible with that sub (ie, everything else).