Q:

Why does SpaceX have a launch window for a pad abort test?

SpaceX has announced a pad abort test for the Dragon crew capsule on May 5th. They have a 4 hour launch window for the test. 
There is no destination, so why would there be a launch window in this case?

A:

There are many different constraints that putting a time window around a launch attempt (even a pad abort) can help manage. Orbital constraints, facilities availability and many more.
In the case of a Pad Abort, consider the flight path. Ignite the Super Dracos, get high enough for the parachutes to be effective, and land in the ocean just off the coast.
That means a Notice to Mariners that they need to stay out of an exclusion area. Thus a launch window puts boundaries on the maritime exclusion zone.

A:

There are all kinds of time constraints on a range.  The obvious ones involve rendezvousing with another vehicle on orbit, but there are other, more mundane ones.  For example, work shifts and sunlight hours.

