It isn't shocking at all in this context - GitHub is for hosting code (and other code-like things), it isn't a content delivery network.

I would guess because Github was not being used as a code repo, but was essentially being used as free CDN for the data file. I've seen direct linking to Github raw files like that before and wondered if it was something they condone.

Yep. GitHub were cool about it, explained that some people were making thousands of requests per second for no reason at all and it was impacting others users on the network. It's not the intended use of GitHub at all and I genuinely had no idea it would become popular.