We can't actually use the "c0" format for our use-case. In the example, the '$' is just a placeholder. For us this is a user-defined value and could be an arbitrary currency symbol, units, etc.
We also rely on conditional formatting to allow for customization of how negative numbers display (e. g. "($10)" vs. "-$10").
As you say, the current behavior is not what devs or users generally want/expect, so I do consider this a bug rather than an enhancement request.
Nevertheless, I'm happy to file a request for this on UserVoice as well.