There was the following passage in New York Times (April 28) under the title, “In Baltimore, we’re all Freddie Gray.”:

“We’ve watched as Mayor Stephanie Rawlings-Blake, in conjunction with Police Commissioner Anthony W. Batts, spent over a week investigating what appears to be an open-and-shut case. I’d like to think that if I broke a person’s neck for no reason, I’d be charged in minutes. But the system — even when it’s run by a black mayor and a majority of the City Council is black — protects the police, no matter how blatant and brutal they are.

As I am unfamiliar with the expression,“open and shut,” I searched the meaning on Google, and found the following definition by the Free Dictionary among several sources:

Open and shut - If a legal case or problem is open and shut, the facts are very clear and it is easy to make a decision or find a solution

Is the usage of idiom, “open and shut” limited to legal matters? If it came from the instantaneous action of opening and shutting something like a box or drawer, why can’t we use it on anything, questions (e.g. simple calculation) or problems that are manifest or easy to solve other than legal cases?