TOTP's, American Bandstand.. it was all a part of the agreement back then.
They had specific time constraints they wanted to go by and to use the show as an advertisement for the music. So, all the bands had to mime. If they refused, I think TOTP's then wouldn't let them play, right? I'm not too sure about that one. The bands that managed not to mime put up a huge fight about it or something else, I would imagine. Then later on, more bands were doing it live or half live.