The reason you don't hear anybody address the specific issue of a DA firearm being carried cocked with the safety off is because it is glaringly unsafe. These guys you are dealing with may be otherwise decent guys but this practice is putting both themselves and others around them at risk.
To the comment regarding the IDPA range commands, this is the way competitions have been run for decades. The procedure is common to just about all disciplines I have been involved with.
- Competitor completes the course of fire, drops mag and shows clear.
- RO checks the chamber is clear, tells competitor slide forward, hammer down, and holster
- Gun is holstered and is not touched again unless the competitor is on the firing line or at a safe table.
Everybody shooting in one of these competitions has attended a mandatory safety brief and is aware of the required procedures. Regarding the holstering with the slide back, not all holsters will accommodate this. I know that my Safariland will not. If the slide is back the front of the gun is not properly supported and is prone to wobble and possibly fall out. The other side is for competitors shooting revolvers. You can't leave the cylinder open on a revolver and holster it, thus the procedure above becomes common to all firearm types with no exceptions.