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.
http://www.chicagomag.com/city-life/April-2017/3-Things-to-K...
tldr: They're rent-a-cops hired by the airport, not United.
However, this is Chicago. Locals know to never assume anything about city government because you'll tend to discover the opposite later on.
Having lived in Chicago before, you fear the Chicago Police and your local alderman is the fixer.
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!
https://www.aopa.org/news-and-media/all-news/2003/march/31/m...
http://www.airspacemag.com/flight-today/day-shut-down-meigs-...
[1] http://www.chicagotribune.com/news/local/politics/ct-united-...