Just to clarify: These Enfamil checks (or Similac, etc.) are real checks -- NOT coupons. The processing of these checks is the *exact* same process as when you write a check: the money is deducted from your checking account (or, in this case, from Enfamil's checking account) and the check is sent back to you after it's been cashed (or sent back to Enfamil.) These checks are "legal tender" and are regulated and protected by all the same financial/banking laws as the checks in your checkbook. The only difference is that these checks can only be used on Enfamil purchases. As far as the store is concerned, the store is not discounting the product -- the store is immediately being paid by Enfamil, via electronic transfer, for the amount of the check. These checks are as good as cash.
Using an Enfamil check written to someone else is the same as forging someone else's check. Which is illegal. That's why the store doesn't need to let you use the check. However, unless the check is expired, the store is usually going to get the immediate reimbursement from Enfamil (the store does not incur any monetary loss)...which is why so many people don't have issues using them fraudulently.
As for coupons and coupon fraud: Coupons are discounts that are given by the store on the good faith that they will be reimbursed by the manufacturer. They are not legal tender. The store deducts the coupon amount and takes the "loss" and is counting on the fact that the manufacturer will reimburse the store. The store sends the coupons/electronic records of the coupon discounts to the manufacturer and waits for the manufacturer to reimbursement the store. When counterfeit coupons are used, the manufacturer refuses to pay -- the store is simply out of luck and has to take the monetary loss. Then the store increases all their prices across the board to make up for all the people cheating and using fraudulent coupons.
Hope this helps.