Comment 5 for bug 1009551

If you think about it for a little bit, I think you can derive to an answer. Usually GPG keys are part of the Web of Trust and represent a person or a role. It is public-key cryptography, and not a shared secret. Launchpad allows privileged operations based on the GPG key signatures alone over otherwise untrusted protocols (dput package uploads), which thus must uniquely resolve to accounts.