It's a Chicago cop who is assigned to the airport, I think.
They're actually
City of Chicago Aviation Department employees, not CPD. They have limited arrest rights and don't carry weapons, though they are certified Illinois law officers.
"At will" doesn't apply to public employees who work under a CBA/contract.
In any case, Illinois is one of the last remaining bastions of union power in the U.S. and is not an at-will employment state.