State Dept on what is considered Antisemitism: https://www.state.gov/defining-antisemitism/
These definitions are intentionally broad and designed to censor criticism of Israel. You have more freedom to criticize the US Government than to criticize a foreign country.
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."
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.