I know exactly what your talking about, and it irritates the living day lights out of me as well.
I recently located a nice out of the way place to shoot that I thought wasn't at all well known, at least judging by the lack of brass and target junk it seemed so. So a couple weeks ago I'm driving into the spot, and I see this SUV, but no one was in it. Once I was set up I didn't want to start shooting until I knew who was where, and then I see these two guys walking out from behind the dirt back stop. The first thing they said before even extending a greeting was, can we have your brass, and just so you know, were going to be in the area scouring for brass. Both of them went on to say that neither shoots or even owns firearms, they just collect and sell brass as recycle metal. I politely informed them that I reload, so no, you can't pick up any brass in my shooting area, as it may be mine your picking up. I went on to tell them that after I'm finished shooting, and have picked up my brass, the area is all yours. They packed up and left with little more being said.
But yes, it does kind of ruffle my feathers a bit. I'm running into more and more of this lately, and it irks me cause good brass is being sold off as scrap metal.
GS