You are not thinking it through. An imperfect analogy to help make the point: You can't put a backdoor in encryption "just for the good guys" because there's no way to prevent that backdoor being used by other actors. Similarly, you can't put restrictions on speech "just for the really bad things" because there's no way to prevent things being labeled as "really bad things"!
"Encryption" is not a place or a thing. It's a fact of nature. It's a mathematical truth. Anyone can take advantage of it once it's been discovered and disseminated. That cat cannot go back in the bag.
The US government is a unique thing that we risk losing control of if we don't act now to defend it. Social media platforms are single things or entities that can revert policies once a situation de-escalates.
I imagine that a lot of the people making these calls at these companies would like not to have to do it this way but feel they have no choice. I believe they would be happy to resume business as usual when they're not worried that a violent movement will overthrow their government. Until then, could you really see yourself sitting by, doing nothing, and hoping for the best while watching these maniacs use your platform to organize their movement?
This is rarely observed. See all of the legislation and policies that have remained, and even expanded, after 9/11. Politicians tend to make laws, not remove them. See the tax code for a good example of asinine infinite expansion.
In normal times, I would agree. Philosophies and ideals like this effectively discourage people from taking action. That makes a lot of sense during times of peace when our asses aren't actively on the line.
However, these are not normal times. We are facing a threat that might just overwhelm our society and rob us of our way of life. We must act now to neutralize the threat or risk going extinct. We can sort out the ethics of it all later.