I agree with you. Protesting and boycotting are honored libertarian strategies.
It would be interesting to randomly poll some non-libertarians at the protest with the question, “is the tech cos. censorship a violation of the 1st Amendment?” My prediction is most people would answer in the affirmative.
Of course you're right that the First Amendment protects against government anti-speech laws, not censorship by businesses or property owners of what happens on their property or in their fora. However this does not make "private" censorship by big tech companies right, and we have a right to speak out in the marketplace of ideas and raise our voices against it.
Love & Liberty,
((( starchild )))