It is sooo dumb, but I think it is more to do with pandering to the electorate than anything. A good few years back, there was a report of some guy using a radio scanner to listen in to the police, find out where the queen was going to be that day, and then post it on the internet. I don't think he was doing it maliciously, but probably not a clever thing to do, and you can see why the security services would be unhappy with this. Anyway, one thing that came out from it was that there had been a plan to introduce encrypted radios, but they'd cocked it up or something.
Oh, and the solution to the problem? Ban radio scanners. You know.... 'cos then bad people won't use them, what with them being illegal.
* Some details may be inaccurate as this is from memory.