I know our pre-release milestone naming is a little inconsistent and at times appears haphazard. We're working to standardize the naming and make it clear to testers that "Beta 1" means "this" while "RC1" means "that".
Regarding this specific milestone naming, there are a couple of reasons we named it RC0.
I know that the answer "because we've done it this way before" is lame, but it's the truth. We've named many first-RCs as "RC0" in the past, so we rolled with that name this time.
2. We're dorks.
...so we frequently start counting things starting with 0 instead of 1. We do this everyday in the 100's of lists we manage. In fact, our prioritizations even start with "Priority 0".