Private organizations are allowed to decide how their property is used, including the type of speech it is used to facilitate. This is inherent to the concept of private property. I don't know why you think these companies are public forums. If there is a public forum in this space, it is the internet itself.

People who dislike the limits of these platforms have a variety of other venues on the internet to choose from, but what they are not at all entitled to is the automatic amplification of their voice by these large but private platforms.