Right. Now let's say you are the one not using github (you use raw git), and you want to send a patch to somebody using github ("not git"). As TFA notes, either nothing gets done (you send-email and the other guy just doesn't see/want it) or something has to give, you have to create a github repository with the result of your patch or the other guy has to learn git-am.
That could be fixed if github had better mail integration (and more generally more hooks for standard git behaviors) and you could essentially create a pull request by git-send-mail-ing some sort of special address @github. And the pull request got its own mail thread (so the initial sender got replies & al without needing to go on github).
As far as I know, that's not the case.
It's a feature that github SHOULD add. There are some people who want it, and these people are the type of people that can become quite vocal proponents or detractors.
More importantly it signals quite strongly that github intends to become a good citizen in the nation of git rather than trying to appropriate it.
Watching Zach Holman's various presentations on Github and how they work interally provides a third one: since Github uses pull requests internally (and new githubbers are probably github users in the first place) nobody has any issue with not supporting `am` and `send-mail` there, so nobody went and built support for that.
Or you know he checks his email and takes the patch. When you use github you are using git. There is no way to commit from the github website, so if they have code on github then they must have interacted with git. Now that we have established they interact with git then they have the capability to interact with the patch feature if they so desire.
And to your point, you cannot add files via github.