This one is a bit trickier, as few things apply;

1. Voodoo island; shooting at zombie cages point blank is prohibited, because if even 1-2 people shoot point blank at the zombies, it will drastically lower their health every island and by the time they get to the last island, a simple nade could kill them. (not fair for zombies at all).

2. As meltdown said, shroomforest level 1, the zombies are spawned with 50K health, and even after you shoot them down, they will regenerate health back to 50K (or did they change this in the v3 version?) So ultimately, shooting zombies will not benefit at all. ALTHOUGH, many humans are not that smart. They dont know this. And many new players do not know any better either. All they see is "zombies in a cage", they don't pay attention to the butterfly (even when you shout it on the mic, or write it on the board). Most children minds are "ohhhh zombie in a cage, let's shoot it".

3. There is no actual rule where it says humans HAVE to shoot the butterfly. I know, every humans bullet hitting the butterfly is needed, trust me I know. But because a human shooting zombie cage is not considered sabotage in completing the map, the humans just do not know better.

Now if a human purposefully gets too close to the zombie cage to get tagged and tag everyone else, slay away my dude.

Thats my $0.02 cents.