Is this a violation of the 1st Amendment or a way to skirt around it?
Or are they doing it under duress?
Of course it's the latter.
The US gov. has been threatening to take down social media companies for years. Do you think Twitter really wanted to upset them now?
Yes? When the CDC says "we recommend you get vaccinated against the flu", and you get vaccinated, you're acting voluntarily, despite the government recommendation.
Also, the guidelines ask you not to write comments like that.