Freedom of speech does not apply to privately controlled products from private companies. Any one of these companies could create any arbitrary rule to regulate their platform and it's not a violation of free speech. The thought is that they operate in a capitalist system where they are optimizing for profits which means they need the most amount of customers so they are trying to accommodate everyone.

The same way all of these companies actively block spam on their platforms to improve user experiences. This is probably a result of analytics in inside Twitter where they realize that it's costing them subscribers and revenue.