When a man's job depends on him apprehending suspects, then he will find a way to create more suspects for which to apprehend.
Police in the United States have never had any such legal duty, and this has been backed by court precedent. It's purely a motto.
Two relevant cases are South v. State of Maryland (1855) and Castle Rock v. Gonzales (2005).
When a man's job depends on him apprehending suspects, then he will find a way to create more suspects for which to apprehend.