NPM doesn't use tags to do semver comparisons. To NPM a tag is no different than a branch or a commit hash. This differs from Bower and half dozen other package managers that do semver against tags.
Bower has a backend that is reading the git repo and converting tags into semver versions. They automated NPMs publishing process. Doing this in realtime in the NPM client would be very time consuming.
> Doing this in realtime in the NPM client would be very time consuming.
Nonsense; this is a very quick operation in git. Even in enterprise situations you're probably not going to have a huge amount of private repositories.