No? The point is to keep the user's system free of non-free dependencies. As a user, I don't want to audit the several hundred packages that a single Rust application might decide to pull in; I'd prefer if such a thing were enforced instead. Otherwise, you might as well go use any of the hundreds of non-free distributions out there.
The Rust/Cargo license is also terrible grounds on which to build a free system anyway.
Strawman, what is absurd is your own reduction. Hopefully you see the difference between running a build/install command that pulls 100+ dependencies from the network that may change any moment without notice versus downloading a standalone software package that is appropriately licensed.
(You should have no trouble peering at the depths of your own stupidity.)
... Not sure if such modification is not incompatible with GPL family of licenses, though, but I am not a lawyer (you're not allowed to combine GPL-licensed product, whether v2 or v3, in ways that create derivative works whose parts are licensed with more constraints than enforced by the GPL, with special carveouts for AGPL)
The Rust/Cargo license is also terrible grounds on which to build a free system anyway.