That's why they are trying to get reform. A websites own rules may not contradict the rules of a country it is operating in. So if America says a website has to allow freedom of speech if it is a large public platform.
Then twitter has to allow freedom of speech, at least in america no matter what their own policy is. and obviously I mean within reason and guidance of existing government law. so no threats of violence, slander, libel, support of terrorism. These are all restrictions that exist under government law and it would be in violation of the US law if they were to do this obviously US law is soft on Twitter.
There were thousands of cases of support for isis and active isis twitter accounts around about 2016 and there still probably are active twitter accounts belonging to terrorists but because of the size of social media they can't really do much about it apart from ban them when they are reported.