If you read CBS' statement, you'll see that they claim they don't air advocacy ads. They will, however, air ads from political candidates or organizations that have been authorized to speak on their behalf. So the real reason this ad fails their litmus test is not because it's an advocay ad, but because it's not being run in favor of a particular candidate. Rather, it's against a candidate.
Nitpicking? Uh-huh. Restricting political speech? You betcha.