Well, Rust is MIT + Apache 2.0, so they can do this. It isn’t copyleft.
Personally I consider it a a shame that rust and it’s ecosystem isn’t at least weakly copyleft (e.g. LGPL or MPL) though there are some good reasons not to use those specifically. (LGPL isn’t not well defined if you don’t use dynamic linking, MPL is younger than rust, but would have been an excellent fit otherwise). And the ecosystem follows the leader for the most part.
But that is neither here nor there, and I’m not interested in arguing about licenses on the Internet. :)
Some additional thoughts (responding to myself here):
A hypothetical person might respond to this “But I have strong feelings on subject X!”
To which I respond: consider what effect your words will have. Inflammatory words will just entrench people’s position more, in both camps. People on the fence are likely to walk away from the whole thing out of disgust (especially if both sides behave that way) or take the side of your opposition.
Reasoned arguments in neutral tone however will rarely change the opinion of those who strongly disagree with you (there is a slim chance). But it might strike a chord with those who are on the fence.
A tendency for debates to get polarised is a general problem in modern society in my opinion, not just in Rust. Not just politics. Not just media. Pretty much everywhere. I have seen it here on lemmyrs.org as well in the discussion of the drama around lib.rs. But I don’t think calling out specific instances would do any good (rather the opposite in fact, as it might easily be interpreted as an attack on the posters person).