I find this to be a bit of a bad comparison. /r/politics downvotes opposing viewpoints - they aren’t banned. And there are many, many other subreddits where you will find opinions completely different from those of the /r/politics orthodoxy.
What makes you believe it then? Anyway, that sub bans people only for violent threats and such, but opposing viewpoints are downvoted a lot. They even have some pro-Trump mods on their team who can see and reverse mod actions including bans.
Anyway you can try an experiment if you care, post on there (with a new account if needed) very partisan stuff, but don't advocate for violence, and you'll find yourself not banned.
Ha! I regularly see posts from people who were banned for saying perfectly benign things on /r/politics, and what they were banned for.