You are misinformed. Local cable systems do NOT place local ad insertions on network shows like Letterman. The ads come from the network or their local affiliate.
Ok...which is why I started out saying I don’t know much about how the ads work....
But, I remember last year a dispute between the local cable carrier and CBS affiliate. The affiliate (WIBW) wanted Cox cable to pay a small fee (around a dollar per subscriber) to carry CBS. The logic was that CBS was only allowing cable to rebroadcast CBS, as a convienence, so customers could get CBS without an antenna...
However, most programs had a certain portion of the commercial space allotted for local ads. Cable was selling the local ad space, without compensating WIBW. Thus, if you switch from cable to over the air signal during a commercial break, you sometimes see identical ads, but other times see different ones.
The same thing happens when I look at cable CBS from Topeka, vs the Kansas City feed...sometimes identical commercials, sometimes different (sometimes one is blacked out).
I just don’t want to boycott Olive Garden, for example, if their commercial ran in one market, because some baseball game had run short and they couldn’t get all the commercials in...and a local program manager placed it on Letterman to get it played before midnight.