I don't think we can argue in good faith that those forks are publicly distributing a modified version of the rust programming language, unless they actually publish releases.
- In GitHub, forks of public repositories are themselves public repositories.
- GitHub repositories can be cloned, which is a form of distribution.
- Therefore any fork that implements, for example, a change to the programming language itself, but still uses the name "Rust", is distributing a modified version of the programming language in a manner that is not allowed.
I sincerely hope that this is not the interpretation taken by the Rust Foundation, but I cannot know for sure. It seems very open to selective enforcement.
Arguing that it's covered because it's distribution requires chaining through a few overly literal definitions to achieve that result, and that isn't likely to be winning argument against a gut instinct of "no, it's just not."
But legalities have its costs as well. is it worth it to go after some random student project with no stars on it? Absolutely not. I imagine that's the bulk of those github forks.