What's wrong with a sovereign nation taking steps to reduce or eliminate the influence of a non-citizen who they feel is acting against the best interests of that nation?

If a nuclear capable country like France decides that someone like Elon Musk is acting against the best interests of their country they can ask him nicely to stop and if he continues they can use force to reduce the perceived threat.

This all seems completely in line with the day-to-day norms of contemporary society as well as historical norms.

He is a citizen of the US and has full political rights. There is only one legal distinction between a foreign born citizen and a natural born citizen and that is that he can't serve as president. France is absolutely capable of using force against Elon Musk up to and including their nuclear arsenal. However, they would need to decide whether it is worse for their interests to tolerate Elon or to detonate a nuke on US soil, and that's a pretty easy choice.