Why are some grocery store coupons are marked do not double?

I always thought the stores paid the extra discount when they double the coupon value. If coupons are issued to increase sales, why would manufacturers care if the store doubled it? Wouldn't that be good for them?Why are some grocery store coupons are marked do not double?
Some manufacturers have agreements with retailers that they will pay if the store does a double coupon day or something like that. If one of those manufacturers has an exception to that policy, ';do not double'; will be printed on the coupon, but usually the store will absorb the cost of doubling it anyway.