Yeah, I think this is due to the Public Interest requirements, which allow, generally, only one channel per programmer to count as PI. In C-SPAN's case, that means that they can only count the original C-SPAN as PI, not any additional channels they produce.
(The law does say that in a hypothetical future world of unlimited bandwidth, once every programmer who wants one has a PI channel, at that time, existing programmers could have more than one such channel. But, I doubt that everyone who wants a PI will ever get one. My understanding is, that the PI waiting list is relatively long.)