Because they blatently knew about it. If they didn't know anything about the belt, then I could understand. But as the undercover guy filmed a comversation with another Yes salesman, they were bragging that the car is a "ticking bomb waiting to go off", and they were waiting for the head to blow. And as for the manager, refusing to honour the "no-quibble 7-day returns company policy" to a young couple who had problems with their car only minutes after driving it off the car lot.
Yes, I have to agree that not all garages do the work that they should, but Yes are selling over-priced cars to people who go to them as they can't get a decent car any other way.