In the UK ATC do not 'own' the frequencies on which they broadcast. The radio spectrum is 'owned' by the government and ATC apply for a licence to use a frequency. There are very specific limitations on the use of that frequency in geographical boundaries and in height. When ATC detect interference on a frequency, from for example a pirate radio station or some moron transmitting on one (
recent example)they inform the home office. The home office send out a team to attempt to triangulate the source of the interference and they take action to shut it down, including legal action.
This is slightly different to enforcing listening in but the law does cover that under the
Wireless Telegraphy Act 1949 (WT Act). If such legislation seems rather heavy the answer is in the factsheet on the link I posted:
"Q. Isn’t it all right to listen as long as I don’t pass on what I hear?
A. No, using radio equipment to listen in, except as provided by section 5(1)(b) of the WT Act, is an offence, regardless of whether the information is passed on.
Q. Isn’t this all a bit heavy?
A. No. No-one likes their private or business conversations to be listened to. Parliament has passed these laws to protect the privacy of radio users."
BD