Note that the crates.io index is just a single git repo that holds JSON metadata about each crate: https://github.com/rust-lang/crates.io-index . The actual code found on crates.io is hosted on S3. The index is an important part of the system, but there's nothing tying it to Github specifically.
Ah, neat! Works out well for them; I assume it's relatively cheap to just serve an S3 bucket on their CDN (though I guess bandwidth costs may rise rather dramatically if Rust ever reaches Node levels of popularity), while not taking on any other operational expenses of actually running a registry.
CDN bandwidth by nature of the reach of Cloudfront will mostly be offload onto the local peering fabrics. It basically costs nothing in per mbit billing - e.g. linx https://www.linx.net/wp-content/uploads/2017/10/Fees_Schedul.... 100gb hand. That's not to say there aren't other major costs in running a resilent edge network which go someway to justify 0.0Y/GB pricing where Y varies from 1-9 depending on location for non sponsored projects. tl;dr - it won't ever be a problem
Note that the crates.io index is just a single git repo that holds JSON metadata about each crate: https://github.com/rust-lang/crates.io-index . The actual code found on crates.io is hosted on S3. The index is an important part of the system, but there's nothing tying it to Github specifically.