nonemmet - apart from the car park, the airport bus/train station, or somewhere around the terminal, where are passengers realistically going to leave their trolleys ? Security at most airports is too tight for the average hoodie to bother causing trouble.
If trolleys are left scattered in inconvenient spots obstructing parking bays, roadways etc then perhaps the airport will be incentivised to encourage people to return them to designated bays by way of a deposit scheme like many supermarkets employ. The UK public are (generally) too compliant, lets have some trolley disobedience!