They are not. They are decided what is published on their platform and what not. Who can publish on their platform and who not. All limited to their respective platform. The term "allowed" in its generality can only refer to legal action, and that is reserved to the state. And I am sure there will be plenty of legal aftermath to the recent events too.

Having said that, it is indeed disturbing how large the actual power of those big companies is. As a society, we have to consider how this should be handled going forward.