TV homes with unwired cable access are referred to as ADS or Alternate Delivery Systems. Originally comprised of four components, ADS is now primarily DBS or Direct Broadcast Satellite homes (aka DISH and DirecTV). In certain markets, advertisers can insert local ads on interconnects which have DISH and/or DirecTV, but there are limitations, including which channels can be inserted on. There are still a number of markets where these systems cannot be inserted locally, and to get an accurate reach of a local cable ad schedule in these markets, the ADS percentage of the audience must be deducted if they are included in cable’s ratings submissions.
According to Nielsen data, national ADS penetration in Nov’18 is currently at 28.0% of television households. Many individual markets have even higher ADS penetration.
Here is a look at an example in the Billings, MT market. When ADS is deducted, ad-supported cable delivers a fraction of the "All Cable" rating:
Source: Nielsen Nov’18 Quarterly Universe estimates