It was ABC, or so I have heard, that had re-used old tapes to record new shows over and this was done to the Cal Jam video tapes...sounds frugal but back in the early 70's, hey, I re-used cassette tapes over a few times!
The BBC were much worse, Music was considered a disposable commodity, and they got rid of many recordings when Colour TV came in because they saw no reason to keep Black and white recordings so many TV shows and Music shows just dissappeared.
Its a legacy they are still from time to time recovering today see these two news stories.
http://www.bbc.co.uk/news/uk-england-oxfordshire-16150178
http://www.bbc.co.uk/news/entertainment-arts-16136521