Understanding Stacking?

What allows one store to allow coupon stacking and not another? I just don't understand this? Any insight into this one...Thanks :)

Some companies don't allow their manufacturer coupons to be stacked because it's more money out of their pocked. Any other coupon used for that one item HAS to be covered by the store. Big stores that have their own coupon (PC) don't accept it because they have their own, obviously, although you can use a manufacturer coupon and a store coupon. If you go to a smaller store, that doesn't provide their own coupons, you may be able to stack them as their way of helping customers.