I do not believe that must carry extends to HD-2 channels or to HD channels when there is an analog equivalent. For example, systems DO NOT need to carry WPVI-HD if analog WPVI is carried. Doing so is at the discretion of the cable system and market forces determine that. As more and more people get HD TV's, the demand grows and more such channels are added.
Nor does "must carry" apply to "low power" channels, such as channel 7. WMCM requested must-carry status throughout the market and got it. Money probably didn't talk at all - the cable systems in the DMA were stuck with them. They are digital only, but they have a full power license, which is how a digital channel got must-carry status.
Channel 40 in Wildwood (not AC, big difference) is one of those funny exceptions where it is embedded within a larger market and only gets status in areas where it is substantially viewed. As it duplicates another NBC network signal, it does not get full market status. The same is true of WMUR (ABC) in Manchester, New Hampshire - which is technically in the Boston DMA. It doesn't get carriage in or around Boston - even though a much weaker WNDS from Derry, NH (now Boston's MNT affil.) gets cable carriage in and south of Boston. But, WMUR does get coverage in northern MA (where the OTA signal is good) and throughout NH - even in places where no Boston stations are carried. Those two cases are exceptions to the rule.
Again, none of these examples involve money talking - unless you're referring to the original cable act that congress and President Clinton approved back in 1996. That thing was a travesty.