Perhaps a better question is why aren’t more people shunned. There are some good answers already, but I’ll throw in another reason that seems to come up sometimes.

The US is highly polarized on a lot of issues. If one can frame their shunning as one of those wedge issues, they can probably get enough people to rally around them that they escape accountability.

“I’m being deplatformed because of racism against white people!”

“We have freedom of speech in this country, so if you take everything in the Constitution absolutely literally like I do, it’s obvious I should be able to spew whatever bullshit I want. You don’t hate the Constitution and the founding fathers, do you?”