That is not unique to github pull requests, is it? I thought that was common to all of git, and the reason why git has separate "author" and "commiter" fields
yes, as they gotta pull from you for that. github just makes the process easier and it is more trouble not to attribute via github.
in both cases you can make a patch (=copy/paste) instead. and in the first case you often actually provide a patch because it might be easier or because they're using something else than a git or git-like vcs
