Seriously, what is your problem? Did a volunteer sleep with your wife or girlfriend? did she leave you for one? the amount of hostility you are displaying regarding this particular topic is abnormally and alarmingly high.
I agree that the community should be supporting this idea, especially as a trade off for the manpower providing the service for free. you could also say they do, in the way of donations to the organization. But I am also a realist, and you can't and shouldn't demand that a business give you discounts or free stuff.
While I agree volunteer does free and not paid, I don't think providing them $20 for a decent meal (and no, dollar general or walmart, especially on the clearance rack is rarely tasty or healthy) is too unreasonable. If you want to go to dollar general for your meals, no one is stopping you, but many of us would rather enjoy our meal than worry how much damage it will do to our GI tract.
If you are being paid to be there, than by all means pay for your own meal out of your own pocket, or pack a lunch. After all, for a 12 hour shift at $10 an hour (I know most make more than that, but it's nice round numbers), you are costing the agency $120. The volunteer, who is doing the same job, is costing the agency 0. Providing them funds with the express purpose of being able to eat (esp a $20 limit) seems to be a good deal for the volunteer, and a great deal for the agency.