I don't know that it's specifically required for a visitor visa, but 'Good Moral Character' is required for naturalization in the US. Activity on social media is probably an indication of moral character, so it's not unreasonable to check social media before issuing visas that have a path towards citizenship. Student visas may technically be visitor visas, but there's a clear path F-1 -> OPT -> H-1B -> EB-2 or EB-3; if you're going to check on moral character at the end of all that, you may as well check at the beginning too.
What constitutes good moral character might not be a great question for a government to decide. There is certainly potential and precident of the government using good moral character as a proxy for discrimination that has nothing to do with morality.
This is absolutely not true.
There isn't a single country in the world with absolute freedom of speech to begin with. And even if we take the very permissive freedom of speech of the US, it is matched by only very few countries, even in the west.
As a simple example, here's a map of the countries where it's at least an offence to insult the head of state: https://upload.wikimedia.org/wikipedia/commons/8/80/Lese-maj...
If such a ruling was later appealed and eventually overturned, then of course that would not count. But a judge ruling on a case is technically the only way something can be considered provably "illegal" in the US.
I realize that you want to believe there are certain inalienable rights that somehow no longer exist, but I do not believe that to be the case. The courts exist for a reason, and just because people are sometimes arrested or charged with things that are not actually legal, that doesn't mean it won't later get thrown out in court, and it doesn't mean that your rights have disappeared.
Even if all of that did happen, such a case would most likely be appealed to the Supreme Court and ruled unconstitutional.
And secondly why would US government target only anti-semits, will they check for anti-white racism, African-American racism, anti native-American racism, homophobia etc. This is a mess of a policy. And Trump is openly homophobic and anti-LGBTQ+, what that should tell us?
Abraham Lincoln said: "At what point then is the approach of danger to be expected? I answer, if it ever reach us, it must spring up amongst us. It cannot come from abroad. If destruction be our lot, we must ourselves be its author and finisher. As a nation of freemen, we must live through all time, or die by suicide."