Hopefully because you have self restraint and are able to see that every group of a certain size has at least one irrational member? There's nothing in holding even most extreme political views that precludes trading others with human decency.
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It's because silence is perceived as agreement.
When some extreme position (perhaps on a social issue) gets espoused by somebody with left leanings, there tends to be little to no pushback except from right leaning individuals. This gives one the perception that there is a mixture of apathy to agreement with said views. Similarly, if one does push back against said views you're immediately bucketed as a right leaning individual, or "worse." So it all becomes a bit of a self fulfilling prophecy.
This is why any sort of censorship, or self censorship, really just ruins (little d) democratic discussion and debate. You have no idea what people "really" think, so you're kind of left to just imagine it. And we mostly get it completely wrong. See the Perception Gap: https://perceptiongap.us/
..both ?
If extremism in some members of a community or social group is not vehemently rejected by that community it quickly becomes their face to the outside, because the extremist elements are also usually the loudest ones. But "gotta stay together" or else "the enemy" will win!
At the same time, if "the enemy" is giving your ammo to shoot them with why on earth you wouldn't use it?