unencrypted radio transmissions are in the public domain
Depends where you are. In the UK, this is what OFCOM says:
"Although it is not illegal to sell, buy or own a scanning or other receiver in the UK, it must only be used to listen to transmissions meant for GENERAL RECEPTION. The services that you can listen to include Amateur and Citizens' Band transmissions, licensed broadcast radio and weather and navigation broadcasts.
It is an offence to listen to any other radio services unless you are authorised by a designated person to do so."
Full text here:
RA 169 - Receive-Only Radio Scanners etc
In practice, of course, I've never heard of someone being prosecuted for listening to, or recording, ATC.