This has a really big security flaw, if the creator wanted to be mischievous he could change his js file to do things to your logged in github account, for example set your privet repositories public. If you want to use it you should fork it and change to bookmarklet to use your fork of the js file.
Though I'd never do such a thing, your point is completely valid.
Its a little bit of a pet hate, the suggested linking to JavaScript files in another persons repository. The worst was the HTML5 Shim, for a long time they suggested linking directly to their svn repo, fortunately they don't now. It was about the same time people started thinking about using the google cdn for javascript libraries and so people just did it thinking they were helping their page load times when in fact they were compromising the security of their users and themselves.