Anyone that's put an AM radio next to a CRT TV knows it emits em noise.
The CRT itself is, to over simplify, a high voltage capacitor. It's one of the most popular devices used to power hobbyist Tesla coils...
I don't know why everyone seems to think that just because TVs were "receive-only" devices, that they wouldn't emit any sort of easily detectable signal.
Also, when this enforcement began, broadcast was the only source of TV content, so the presence of a TV was generally proof of watching broadcasts. There wasn't really any need to prove the TV was tuned to any particular content.
Now, whether it is a practical, effective method of enforcement that was actually used is another matter.