Also, TV is relatively easy to monitor. With radio every market has many stations that, at any given location, can not be received or received reliably or received at night, etc. It would take multiple locations to check all the signals in most PPM markets. In the LA market, there are even multiple cases of the same AM or FM frequency being used twice within the market (94.3 FM and 1220 AM are two examples and there are several more).
Good point. Nielsen has several monitoring points (last time they called, it was because of a problem at a cable system at the eastern edge of the market -- Nashville itself was fine) but they don't need nearly as many points to monitor TV as they'd need for radio.
The solution would seem to be an encoder alarm with "you have to see it" notification in the studio and engineering... but with so many stations unattended at certain times, even this is not perfect. Messaging for the key staff is another useful mechanism, but the staff has to know how important this is.
In the end that kinda falls into the same category as having someone check the station's own decoder periodically -- us techies can provide every tool under the sun but we can't force the staff to use them