Well, that's totally screwy and stupid IMO. Tough situation for the kids with parents being split up (been there, done that as a kid), but that reasoning is just stupid. I could somewhat understand if it were a safetyconcern or something like that, he was concerned with, but that doesn't sound like it at all.
To me, it doesn't sound like he has the kids best interests in mind. If he did,there would be something more substantial tohis reasoning than what he's giving you/them now.
I guess just try and explain it to the kids the best you can. Hate to see this situation cause more grief between your family, and would especially hate to see maybe the kids give up hunting because they don't want to choose between one or the other. Smack your ex with a frying pan and hopefully knock some sense into him.

Good luck.