This is the central myth of “free speech” under the 1st Amendment.

The 1st Amendment only protects individuals exercising speech and expression from laws being passed or used to silence said speech or expression and from agents of the government abridging the rights. That is a lot, but it is also all. Private citizens are allowed to abridge each other’s speech as much as we like. Private companies can censor whatever they want. This is why the guy wrote a letter to Steam, because literally ANY larger action would have crossed the line.