Every shoot I’ve been to, whether large or small, has had a rule you can not pick up hulls once they hit the ground. More than likely it’s a rule caused by shooters doing dumb stuff like picking up the hulls in between boxes.
People get really crazy about hulls. Just go to Walmart, buy STS for less than 7 bucks a box, shot them, and now you have great once fired hulls. Or buy them for the vendors here for a nickel each. Trap shooting is an expensive hobby, and I just can't see the cost of hulls making a difference.
One solution that has worked for me at some shoots is to just make a deal beforehand w/some of the trap kids to pick 'X' kind of hulls up for you and get w/them afterwards. I think the rule(s) about not picking up hulls during a shoot are for both safety and to keep the flow smooth. And those rules have been in place for years at numbers of shoots.
W/all the Wuhan virus mandates in place at present, it is more than challenging enough to run shoots efficiently and no one would welcome any activity that would further slow things down, but it won't interfere w/anything if you can do as suggested above. Just saying.