Saw on TV news this morning, those weren't even real cops. They were private security hired by United. They were even told to stop wearing jackets that said "POLICE" months ago, but kept wearing them.
They were Chicago Department of Aviation police. They're hired by the City Aviation department and usually have police training, but are not allowed to arrest citizens or carry guns (although they have been asking).
If you're the mayor and you want to close an airport, do you follow FAA procedure and give notice to the government and all the aircraft owners (especially the ones with planes parked on the tarmac)?
Nope! You shut it down in the middle of the night and bulldoze the runway. Problem solved!
Not necessarily.[1] Also, Illinois law doesn't give local police state-wide jurisdiction. (Some states, including California, do, but Illinois does not.) Unless they're off-duty Chicago cops, they don't have any more authority than ordinary rent-a-cops. They could be charged with assault and battery.
http://www.chicagomag.com/city-life/April-2017/3-Things-to-K...
tldr: They're rent-a-cops hired by the airport, not United.