Twitter does not provide a private webforum, it is open to the world to see and read. And it's not just about the USA it is globally accessible. When operating a company there isn't a single regulation nor a single enforcing agency, in order to be compliant there are many different ones, not just in your own main registered country but also in many others. Those vary from how you account for your finances, deal and treat your staff, but also cover equality, anti-slavery, privacy, and the list goes own.
Just having a 'private' company, and a terms of service doesn't overrule such compliance obligations, in fact in many jurisdictions it doesn't really matter if you make users sign up to your TOS as you can't take their rights away.
The USA is only a small part of the world, and yes employee and consumer rights aren't the best, so I wouldn't use that as a yardstick for the common denominator.