Why would a kid have them? Why would a kid have any guns or ammunition? That's not really the point. Boys don't need much prompting to play with guns if they, by some accident or sheer neglect, gain access to them. I think we've done a lot to educate kids, and that firearm safety is probably at an all time high. But, again, this is not the point. Sure the ultimate responsibility falls on the store owners and the parents. But that doesn't mean the company bears no responsibility.
So, then, why manufacture a brand of ammunition that is labeled something that would catch a kid's eye in a way normally associated with fiction and fun? The label makes it clear, this is not a toy, you might respond. But those labels only do so much. If labels were foolproof in preventing gun accidents, there wouldn't be any. This gets back to the idea of prompting. If a boy is really curious about a gun that has been left out, and he sees a box of bullets with the word zombie on them, he might associate it with playing, with fun, with pretend. And that might be just the thing he needs to think it's okay to mess with. Or at least enough to override the part of his brain saying "no", and go with the side saying "yes."
Now, obviously this would be a very rare, very specific, not at all universal thing. Most kids who find a gun lying around will not play with it, and there are some who would undoubtedly mess with it regardless. But with tragedies like this, it only takes one incident to make a huge difference. So why risk it for a cheap marketing gimmick? If just one extra kid sees the word zombie and that makes him think it's fun and games as opposed to serious business, that's a problem.
It's not like this product offers any extra benefit to the hunter, the plinker, the guy using it for personal protection. It's purely a joke. And as I am sure most gun industry types would tell you- there's nothing funny about operating a firearm.
Ultimately do I think this should be outlawed? Not necessarily. But do I think it's something a conscientious, responsible company would market? Not really.