The owners of printing presses have even more power, but the government does not mandate open access to those machines for dispensing speech. Unfettered access to twitter is not akin to free speech.
Twitter would not be mandate to open up access to everyone. Instead, the proposed legal changes would merely treat them as publishers, if they act like publishers, and therefore twitter would not have liability protections anymore.
If they don't like the section 230 laws, that already exist, then they should go lobby Congress to change it or something.
But right now, if you act as a publisher, then you lose certain protections. That's how that law works.