...can you point me to where it says it’s actually illegal?
It's not
illegal of course, a mistake still often inexcusably made in the press. The linked page from the Dartmoor Park site is stating the 'official' position with regard to CV rules and guidelines that may be enforced by the police.
Come to think of it, I don't know the legal status of this and similar CV rules. Most have adhered to them for the greater good and the police have taken action in many cases, but there have been questions about the extent of such powers.
Edit: Ah, I see Richard has provided more detail. Also by "not illegal" I meant generally, before CV hit us.