I guess the answer to this question is: Because they can. TV watchers would be furious if they had to buy a set top box to decrypt something that used to be unencrypted.
I'm pretty sure that this is exactly the case in the US, at least. You do need to buy a box to watch over the air broadcast after the digital transition, although technically it is probably encoded rather than encrypted.
That's actually not the case in the US. You don't need a box if your TV has an ATSC tuner. The only reason a lot of people need boxes is that they have old TV's with NTSC tuners.
