Because when the cops measure your speed they can do so in an arbitrarily small time period of their choosing, and inertia and power requirements mean you can't be going 1000 MPH one milliseconds and 10 MPH the next.
You could decide to measure APM by saying that the time difference between any two actions extrapolated to sustaining that rate over a minute or seconds couldn't exceed the APM or APS, but as I've explained such a measurement would unfairly give the human player an advantage because humans are capable of bursts they couldn't sustain over longer periods.
You could decide to measure APM by saying that the time difference between any two actions extrapolated to sustaining that rate over a minute or seconds couldn't exceed the APM or APS, but as I've explained such a measurement would unfairly give the human player an advantage because humans are capable of bursts they couldn't sustain over longer periods.