What they do is they have a database of all addresses in the UK. They match that up with a database of all addresses known to have a licence. Those that are left, they figure, must be watching TV illegally.
They assume that you are guilty until you can prove you are innocent. That would be like the police assuming everyone driving a car has stolen it until they produce their paperwork.
Even if you reply to them and say that you don't have a TV set and therefore do not need a licence, they won't believe you. They will still send someone around who will try to enter your property and verify for themselves that you are telling the truth.
As far as the law is concerned, they have no legal right to enter your home unless they obtain a search warrant and they can only do that if they can convince a magistrate that they have reason to believe you are watching TV illegally.
You don't legally even have to give them your name. I know all this because I've been driving them crazy for six years by refusing them entry and refusing to give my name. I still get the threatening letters, but I ignore them. If I was to reply I'd have to pay for a stamp and they'd STILL require me to let one of them into my flat to prove I didn't have a TV.
