Then don't travel, in many countries the penalty for disparaging the leader of that country is quite severe.
Here's one example: https://www.theguardian.com/us-news/2025/mar/19/trump-musk-f...
It also sounds like the guy was actively and openly trying to convince researchers to leave the US for France. May have even been the purpose of the trip.
Free speech is one thing, extending a visa to foreign individuals that are actively working against the country's interests is another. Few if any countries would grant visas knowing that intent.