You seem to be conflating what does actually happen, with what we're all saying should be happening instead...
The actor may well have no such requirement in his contract... but therein lies one of the biggest follies - Everyone who lays a hand on any weapon on set should be required to complete a safety course. The fact that it isn't a requirement is what we're suggesting should be changed.
As for how simple it is - I could teach you the entirety of it in about two hours. Less time, if you're more willing to learn than the average teenage kid.
They aren't... and THIS is the very point we're saying should be changed.
In any other situation where firearms are handled, ultimate responsibility rests with the person who pulls the trigger, even if they're in training and it's their first time. Acting should be no different.