Free speech only protects you from getting arrested by the government. Any private company can ban you or kick you off their service. This is probably one of the easiest thing to understand and people who swear by the Constitution are forever confused by it.

Maybe this will help you. They made stick figures to keep it as simple as possible.

Also, if your speech incites a riot, you can be arrested for your “free” speech.

And when you say “how dare Twitter ban someone” and compare it to Stalin or Hitler, this is a false equivalency. If the internet was around in Nazi Germany or Communist Russia, you would be arrested for your tweets, which is what the 1st Amendment protects you from.