> For executable applications it makes sense to not want GPL only when you want to extract parts of them and insert them into other programs.

It is very common for applications written in Rust to be split in multiple reusable crates. Looking at the main crate, that is the case here too: https://crates.io/crates/coreutils/0.8.0/dependencies

This allows for the learnings of uutils (and by extension GNU coreutils) to be able to be leveraged by any other project that needs the same functionality. I noticed on a quick scan of the dependents on uucore that other projects (like nushell) do so.