Man of Honour
They don't want a pay as you use model. If that happened, their income would decrease significantly unless they increase the licence price. But then they would start having to compete with commercial media organisations.
They just want as many households to pay as they can. The best way to do that is just give everyone access and the realisation that most people won't be able to help themselves.
It is why unsolicited goods rules exist, but this happens to be an exception.
Requiring a logon doesn't require a pay as you use model. They could use exactly the same model, but just require verification of a TV licence as was suggested.