Piss poor management of the players. That's an awful thing to say to your employees, especially when there are ways to mitigate or manage them deciding to do *expletive deleted*it that makes your franchise look bad. The insinuation that they're inmates means that you either think you own them, or that they're nothing but a bunch of criminals.
The phrase he used is something called a "figure of speech". There are many sayings as such that have been used...some for generations. If one has half a brain...oh wait, they're football players who have been coddled and catered to since about age 12. If a player is insulted by a comment/adage/figure of speech like that, especially when you are an employee who has engaged and is engaging in behavior that endangers the bottom line of a business, embarrassing your boss, you are not only a moron for publicly taking umbrage, especially at your pay grade, you should be an ex employee.