US citizens do have a right to listen to the opinions of others under the first amendment so if the US government does have a visa program then it needs to be run in a way that is viewpoint neutral. so the US government can't revoke the visa's of people who have controversial opinions. of course the funny thing is being lectured about this by people from Europe or other countries like NZ, AU, CA or UK where it is common place for the government to revoke or deny visas for people who have the wrong opinions.