You're example is false equivalency. And what you call a preemptive ban, you'll actually discover that they are quite used to being brigaded on that subreddit and they know who participates. And bans aren't necessarily permanent there.
Having rules that say "we don't value free speech or discourse" doesn't excuse them.
Also, reddit allows you to subscribe, but twitter creates a stronger relationship between poster and followers. You'd be subscribed to a particular person but when they are banned, all their history is gone, you no longer see anything. It's like someone came into your house and took away one of your favorite TV channels, as well as anything you had recorded from that channel on your DVR.
Who is this THEY you keep referring to anyway? You keep saying 'they' like readers are just supposed to know.
As a live-right-now example, the mods of /r/The_Donald on Reddit are preemtively banning users before Trump's AMA to prevent dissent, and have stated plainly they will delete any unflattering questions.