How can discount food stores accept manufacturer's coupons?

haha just out of curiousity! i have a discount food store (with dented cans, out of date toiletries, other random kitchen utensils, etc.) near my house, and it is obviously really cheap. but it still accepts manufacturer's coupons that come from magazines or newspapers. is that a law? do they HAVE to accept the coupons? cause i could pretty much get all my food for free, then. i just found a coupon for 1.00 off salad dressing, and at that store salad dressings ARE only a dollar. i'm just wondering if anyone knows how, financially, they can do that, and if it's like a law or something