Github is exposing public SSH keys
github.com
github.com
Also, it doesn't make a difference, since they are public keys, like public GPG keys. They also aren't the only ones that do this - LaunchPad.net (where Ubuntu development takes place) also does it.
This is only an issue if 1) Users are relying on github as a trusted source of public keys, and 2) malicious users can modify the public keys.