He might be catching fish to use as Dead bait or to take away to use as live bait.
He may have several more rings of the keep-net in the water than you think and causing no harm to the fish what so ever.
They may only be very small fish of just a couple of ounces or less.
He may be keeping some fish fresh for the table before knocking them on the head.
He may be keeping his beer cold in the net....lol
There are many possibility's of what could be going on here and I think people should not be so quick to judge or condemn what other people are doing with out knowing the facts.
As I said you can all presume as much or as little as you want and continue to slate the guy for doing something that you want to think he is doing not for what he is actually doing. If your happy to do that then continue to do so.