Well, neither channel has must-carry status in Philadelphia. The WMGM example is much like WMUR Manchester, NH (technically in the Boston DMA). It's only required to be carried in areas where the over-the-air signal gives them a significantly viewed or local status. That's not the case in Philadelphia proper, where WCAU is dominant and WMGM is not.
In the WMUR example, it's carried throughout New Hampshire, but only on cable systems located in areas of Massachusetts that border New Hampshire. It's OTA signal is much stronger in the Boston suburbs than WMGM's is in the Philly suburbs, yet it is not deemed to cover that area and is not offered on most cable systems.
As for Dish, I believe what happens is a quirk in the regs where all stations from within a given market can petition to be carried. Once they're in, I suppose that they can be offered as if they are in-market. The technical capacity of the satellite provider may not be capable of filtering down to that level of geography. So, they offer all stations within a market. It just so happens that WMGM (like WMUR) is a longstanding quirk, so some folks get two network affiliates for the price of one.
Of course the flip side is for those living near the edge of a market, like folks in western and central Chester County. There, Comcast offers WGAL because its significantly viewed. But, I do not believe that the satellite providers offer anything more than Philly locals. I have heard something about a way to petition for adjacent market signals, but I don't know how that works and I think you'd be forced to jump through some hoops.
As for subchannels, there's no must-carry provision for them. CBS would have to play hardball with Comcast to get 17-2 included on the lineup. Maybe that will happen the next time the franchise agreement is renewed but, until then, Comcast is not required to add that channel.
