And if the artist had bought that storage, and then the storage company said "oh, we burned all your art for a terms of use violation," the artist would be rightly outraged.

Sure, he's not paying, but Google doesn't ask for payment. They provide a service which is backed by Google's implicit guarantee that their web hosting is safe and reliable, and they've broken that guarantee. Google may not have a legal responsibility to not delete everything this artist made, but every time something like this happens, the rest of us should notice it and reconsider whether Google is a good platform to trust with our work.