It seems like dockerhub requires a lot of bandwidth...lots of people being able to pull gigabytes worth of images everyday. Does anyone know anything behind the economics behind this? How can they offer it for free?
You use VC money and run at a loss while focusing on marketing and tech evangelism, getting more and more startups and hopefully established companies using your software. As the cracks begin to show those growing organizations have too much tied to your system and they can't afford outages and need to scale. So they pay you for the Enterprise version of your software where you actually fix all of flaws present in the community version.
Look at MongoDB if you need a good case study. It was incredibly hyped from about 2009-2015, people would defend it in heated online arguments, and today it's rarely considered for greenfield projects. But they're making about $100M/qtr selling subscriptions to Enterprise & Atlas servicing the technical debt established during that hype cycle.
It's not just Docker hub, there's services like the various Programming language package repos (npm, rubygems etc) and the Linux distro package repos.
I would have had Github in that category, but now it's owned by MS, presumably they don't have many of that kind of funding problems...