The assumption that Disney is actually trying to save YOU money by offering the dining plan is the problem.
They're not. They realize that the majority of people who purchase the dining plan will not maximize the value, and even people who "save" the marketed 20% often do so by ordering everything included on the dining plan, even if they're not going to eat it. If you choose to only order what you're going to consume, then the chances of you losing and Disney gaining money go up.
For example, on my honeymoon, I calculated that we did "save" 20% by purchasing the dining plan, but those "savings" were largely made up of desserts and beverages that we may not have ordered had we not been on the plan. Oftentimes, we only took a bite of our dessert, or left food on our entree plates in order to "make room for dessert." Even though there was food wasted, Disney probably at least broke even with us as far as the dining plan went. The cost to actually produce the meals is a fraction of what they charge for them. They may have gained a little more than they would have had we not been on the dining plan, because I might have been less likely to book quite as many table service meals. I came from an off-property "sack lunch" family, so the use of the dining plan helped me see some of the finer elements of dining at Disney. Now I'm hooked, and we average a table service meal a day, even when we're not on the dining plan.
There's also the consideration that many of the desserts we ordered may have been given to us complimentary, since we were on our honeymoon. In that respect, we pretty much broke even.
"Free" Dining is even more of a money maker for Disney, because it's probably more likely that people won't be as concerned about not using all their credits if they haven't actually "paid" for them. In my mind, it's inflated savings that are only valid if there are no other discounts available at the time.
OP: Disney's policy does not permit you to do what you wish to do. If you want to "treat" your family, you should pay for them. If this bothers you, the Dining Plan probably isn't best suited for your needs, and you should forgo getting it altogether.