Here's my take, and I've stolen a lot of it from others who I can't remember, so I'm sorry, but credit goes to you... whoever you are...
Firearm manufacturers spend millions of dollars to make their products as ergonomic as possible. This means it is absolutely natural to place your finger on the trigger. If that is the case (and it is) then it is unnatural to keep your finger off the trigger. Other than picking noses and pulling triggers, almost everything we do involves contracting all four fingers together. Then, as instructors, range officers, range officials, line safety officers, etc., we expect that with a simple pre-match, class, session, etc., warning of "keep your finger off the trigger", people will simply follow that instruction with more ability than any other skill we'd teach.
That is crap.
We need to make our instructions very clear. We need to contribute to good habits by not only enforcing them, but encouraging and reminding them whenever possible. This means that a "holster" command could easily include "with your finger straight on the frame, safely holster". We also need to stress that trigger finger discipline is a skill that necessitates deliberate practice (ie: Slow deliberate holstering) as much as any other firearm manipulation skill.
If we're doing nothing but a safety brief at the beginning, then we should also tell people they need to shoot a 2.5 second Bill Drill, and if they can't... just go home.
I'm not saying personal responsibility isn't important, or that safety shouldn't be "enforced"... It absolutely should, and violations, especially grievous violations should be dealt with firmly... but knowing the dynamics of how fingers go on triggers, we can do much more to prevent the issue with a little creative problem solving.