I was amazed anyone tried to make a free Docker registry. It's like making a CDN, except instead of individual files, it's for a whole app and all of its dependencies. It's a crazy amount of data for storage and bandwidth.
Like CloudFlare? Freemium can be a very successful business model.
Now that they have it, the cost they are willing to pay is based in part on cost-of-switching. Which is pretty enormous, directly and indirectly, when entire ecosystems based on docker have been iterated.