Earlier quoted context omitted.
It's my impression that they really don't want to be a CDN. One might suggest that as a service that could generate revenue if they charged for it, however none of the cheapskates using GitHub as a CDN would ever consider paying for a CDN...
I can't blame them (being GitHub). Bandwidth and storage can be expensive. That being said, I think it's inaccurate to characterize the people using GitHub as a CDN uniformly as "cheapskates". Many are just open source developers, often of limited means, trying to expose their work to the largest audience possible.
When you just start your project you may use github as your CDN and hope no one notices, but when you are as big as homebrew using someones elses resources like this is a pretty bad practice.