All that said, nobody has a "right" to come to the USA to study. It's something we allow, for a lot of good reasons, but there are doubtless a small number of people that we would not want here.
Would you be ok with that social media poster being granted entry into the country?
Do I trust the government to police opinions? No, especially when there's no accountability and appeals process.
Do I believe the overall benefits that harassment-free international travel brings to this country outweigh the costs of letting in some visitors whose views I disagree with? Yes.
However, countries may, depending on their laws, choose to not let certain people in on conditions that would otherwise violate guarantees on freedom of speech etc.
However, you do have your constitutional rights at the border etc. There is an exception concerning searches.
There are a lot of times the government is limited even dealing with foreigners abroad (in legal theory anyways, ymmv in reality).