Once fired brass?

I'm a bit confused, maybe someone can point out what I'm missing here.

I've been considering getting involved in a group buy for ammo on another board. The company that's offering the ammo is offering plinking loads that are utilizing once fired brass. The person coordinating everything mentioned that the once fired brass will not be re-loadable. The headstamps will be mixed and they will be boxer primed, but "Once fire brass has already been loaded twice, if you want to reload it then thats up to you. it will be boxer primed tho.
That said the once fired is FOR THAT LOAD ONLY. ALL OTHER BRASS WILL BE NEW BOXER PRIMED STUFF" is exactly what he says.

Why wouldn't I be able to reload the stuff that has already been fired twice by the time I get done shooting it? Is there a safety hazard or brass damaging process here I'm missing? I bought a bunch of once fired brass and have reloaded 4 times (making a total of 5 loads in that particular piece) without any problems. Am I at risk of something going BOOM?

TIA.

Cool, so I'm not crazy in wondering why it wouldn't be re-loadable. Thanks for the help.

I would probably be one of those who consider post-tumbling no big deal, seeing how most commercial ammo bounces down the road in a semi for a few days. I've heard that a lot of manufacturers post-tumble, but I don't know for a fact. I tumble my brass before loading it though.

Hi

What caliber is it?

If it's a caliber that feeds machine guns, that may be the reason they are limiting the number of reloads. Some (but not all) machine guns can be pretty hard on brass.

Bob

7.62x39, .45 ACP and 9mm.