I mean honestly when you're running plates it's still your responsibility as a runner to check the ticket against the plate to make sure that the cook didn't screw up. The cook shoulders most of the accountability but if you are negligent in checking the ticket you still get in (not the same amount of) trouble. A visual check of the contents before the act of handing over the thing that could be Not Correct is not really the same as "learn to pack your own parachute."
That said, I agree that in the context of guns and props liability/responsibility falls on the team whose actual job it is to safely prepare these prop weapons. I DO think it is negligent to not teach actors to do a final chamber check before going to town because people make mistakes. These mistakes shouldn't be happening in the first place if there isn't live fucking ammo on the set in the first place but I've never worked a job where I didn't have a slice of accountability in the chain before it was given to the client or customer.
Let's say I'm a runner. I didn't take the order, I'm not in the kitchen, I'm just bringing food. If the waiter wrote on the ticket "allergic to ____, hold the ____" and the chef neglected to follow instructions, I, as a runner, cannot reasonably just be like WELL IT'S THE CHEF'S JOB TO READ THE TICKET when I hand off the plate that has the thing the customer is allergic to in it. It's both of our jobs to read the ticket. There's more accountability on behalf of the chef, but we both share in the accountability.
Yeah, it's the chef's job to read the ticket right. You still check tickets in case the chef fucked up. Yeah, it's the assembly line's job to read the work order right. You still check the product they hand you against the W/O before handing it to the client. This is "visual check that things line up" and does not require dis-assembly or re-assembly and there's no ... Occurence of a slippery slope of scope creep for responsibilities on behalf of the runner or CSR.
So while where the "line" is is not clear to me here, it does fall somewhere along the lines of "If what you are doing poses a threat to people other than yourself". If you're the one who stands to get hurt, no, you should not be expected to know how to guarantee your own safety. If you're part of the chain of potentially posing a danger to someone else who is not yourself, you should be expected some minimal training.
Pointing a gun at a thing (ricochet risk) or person counts as "potentially posing a danger to someone else who is not yourself." I don't think it's unreasonable to posit that actors should be trained to do a chamber check because I don't think it's unreasonable to posit runners must check tickets in the window before handing it to the customer. For all I know, they typically are trained to do a chamber check and this just wasn't happening on Rust.