I'll respectfully disagree with that. Check out information on the Henry Mountains herd in Utah, which was for many years the only bison eligible for Boone & Crockett entry (because other "herds" were raised/hunted in the manner you mentioned). From what I can tell, bison hunts in Arizona are also pretty challenging. The rub on either of those is getting drawn for the hunt...I've got 8 points towards a Utah hunt and I won't even begin trying to get a tag until I've got 4 more.

I do agree, however, that there are far, far more pen-raised slaughters than wild hunts when you're talking about American bison.