FWICS, the package is bundling a bunch of crates with various licenses. However, they are not listed in LICENSE. Furthermore, since it apparently produces ebuilds with incorrect LICENSE by design, please either fix it or p.mask it with appropriate big fat warning.
You're reporting two issues in one bugzilla. 1. The LICENSE doesn't include dependent crate licenses, which you should create an upstream ticket for. 2. The format of Gentoo license isn't correct by design due to the above. This forces folks to edit the ebuild before using it and verify the license information. Until 1 is fixed then 2 won't be fixed. Since this an upstream issue, I'll mark it as such.
Crates carry enough information to resolve this. It's integration problem that all this information is discarded, not upstream's.
Please create an issue in the upstream bug tracker for cargo-ebuild. Hence, closing this as an upstream issue.