Its theft, simple as that. Petty theft.
I was at a a public range a while back & had a similar issue. When I firmly, but politely asked him to stop taking my brass he called the (non-resident) ranger to complain. In his, the brass picker's, viewpoint after it left my ejection port it was "thrown away" & so became "scrap, public property".
I wasn't quite as polite as you were.
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Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”