At one point, Mozy which was another backup service acquired by EMC went from unlimited to flat rate up to some limit and then charged incrementally, effectively "firing" their heavy users. I wasn't even that extreme and it made Mozy very economically unattractive to me.

Backblaze knows the numbers and I don't but it's certainly possible that the heaviest users don't cost them that much relative to the benefits of being truly unlimited without much fine print.