I mean, a quick search of /r/politics, which is liberal, suggests to me that you're over-stating the case. Conservative posts are often unpopular and people expressing conservative views get a lot of downvotes, but it was simple to find posts that go against the culture of the subreddit[1][2]. I wouldn't recommend /r/politics as a place to discuss politics - I agree it isn't representative of the median US voters' view.

I think it's one thing to say reddit is mostly people'd be leftists, which I agree with, and another to say that conservative opinions are explicitly banned. I think conservative opinions are unpopular and the imbalance makes reddit a poor place to understand the politics of the median person in the US, but non-leftists are allowed.

[1] https://www.reddit.com/r/politics/comments/fdpixv/city_of_mi...

[2] https://www.reddit.com/r/politics/comments/f8cdys/bernie_san...