I’d assume it’s so no one will go into a grocery store with a “75 cents off” coupon and demand to redeem it for cash. If you’d have to collect twenty of them and then you’d get only one cent in exchange, well, you probably wouldn’t bother.

@Jeruba That’s what I thought at first. But the coupon clearly would state “75 cents off on the purchase of one tube of toothpaste”, for example. It would seem to me that this would preclude anyone from redeeming it for cash.

The reason is “you have to buy something” to get the 75 cents off, the value of the piece of paper if you don’t buy the item is – - ½0 of a cent.
Many years ago groups would try and “cash in” the coupons for cash with hundreds of coupons.

To answer another way, then: because somebody—probably the Legal department—has insisted on the inclusion of that language to avoid some sort of risk.

You can be sure that the potential risk is something that can be expressed in monetary terms.

The “cents off” language might preclude anyone from actually cashing in (and of course it would prevent any sensible person from trying), but it might not preclude some nut case from making a hell of a nuisance of themselves first, maybe even to the tune of a frivolous lawsuit of some sort. The company (any of the companies) will not feel like spending its money or its employees’ time that way, regardless of whether the claim makes any sense or not.