Don't know very much at all about rocketery but, if a test involving the live firing of the rocket is required, would it not be prudent to carry out this test before the valuable satellite is placed on top?
The problem is, the satellite is mounted on the rocket as part of the 'final assembly' process. I believe Space X does final assembly horizontally, then raised the complete rocket to the vertical. So the only way to do the final checks is to have the launch vehicle fully assembled - including the payload. Otherwise, to mount the payload it would be necessary to lower the rocket back to horizontal, which would effectively nullify the 'final' checks already performed.