It is one of the fundamental commandments of gun safety, in fact. The basic reasoning is there are only two reasons for putting your finger on the trigger of any firearm, regardless of condition:

1. To knowingly and willfully discharge the firearm at an intended target.

2. To test the functionality of the firearm's action, after first clearing the firearm to ensure it is not loaded.

Pulling the trigger on a firearm to check if it is loaded is an unacceptable and illegitimate condition check. When I was in Baghdad, they had discharge barrels at certain checkpoints and facility entries. The purpose of these was to safely disengage the safety on an unloaded gun. You only put the muzzle of the firearm in the aperture of the barrel and pulled the trigger after you had already cleared the weapon. The barrel was a required safety device in case of human error. Lord help you if the gun discharged into the barrel and the sergeant heard it. But if he heard the sear disengage after you passed the barrel, it was your a$$.