What is and what is not protected speech has been pretty clearly defined over the years. Honestly though it doesn't matter because these are private companies.

Parler has failed to moderate extremist content and is acting as a platform for radicalizing new users.

Apple, Google, Amazon, and Twitter have a decided independently that the lost revenue and maga tears are worth not having to host far right terrorists.

Parler can host itself if it wants, nobody has made it illegal to be a gaping asshole.