And the people who run the "non free speech" platforms (i.e., all the popular ones) are operationally "opposed" to free speech primarily in the sense that they want a pleasant platform for advertisers, and hence must stifle the stuff that's unpleasant to advertisers. There is some personal ideology at the top of these organizations, but it is extruded through a profit seeking filter and often barely survives operationally intact.

I don't say this to excuse their actions or say that this filter is good, but that seems to be the fundamental mechanism.

Not strictly true.

Those whose focus is overtly offensive and oppressive speech, however ...