People who deviate from group-think are banned in real-life too.
People have real discussions all the time in public, at bars, at work - they are not banned, there is real conversation and people listen to each other (to a point). Even if you end up totally disagreeing with someone, you still carry on with them as an co--worker or a friend - I have many friends who are utterly in disagreement with a number of things I believe in, but we both just carry on knowing the other person is wrong.
If you don't know anyone who disagrees with you, then I feel very sorry for you indeed. But that's the lives of most people, no-one agrees with everything someone else thinks and we all learn to get by with it.
The difference is that online people are simply less tolerant, and vastly more closed minded. It's a shame to see Twitter codify that and bake in a preference in a matter that people are very much split 50/50 on.