I think we can pretty much all agree that taking someone else's brass while that person is still shooting is out of bounds, because they might be interested in reloading it. It would be like hanging around someone else's picnic and picking up their empty soda cans off the picnic table, when they fully intended to take them home and get the refund for them. Yeah, they're "done" with them, but they might still want the empties for later use. I think you're perfectly within your rights to turn to someone who is violating this rule and say, "actually, I am going to reload that - can you please just leave it there?" Not confrontational, just stating the facts.
However, I think we can also agree that brass which someone puts into the brass can is fair game. In the story above, it would be like someone throwing the soda cans away - they clearly don't want them anymore, and so anyone else who wants them is welcome to them.
Where it might get tricky is if someone pours a dustpan full of brass into the brass can and two (or more) people both walk over to comb through it. But that probably doesn't happen especially often.
