The ATF says so. Actually I'm sure they were just recovering from too much A at their post-SHOT party. They finally got their shotgun thing out that they referred to last week.
Just skimming over the begining, it looks like they tried to more narrowly define "sporting purposes," and decided that the only legitimate ones are the ones that have been around for 100 years and only use pretty shotguns that don't hurt feelings. Oh, and "plinking" isn't a sporting purpose either, since you can do that with any gun. So any "action shooting" like IPSC or three gun stuff is not a sporting purpose.
That's all sorts of crap...
Anyway, Uncle's got a better breakdown going, I'm picking up a few extra hours at work tonight, so I'm probably not going to be able to get through it.