Why do I get charged taxes when paying with EBT /SNAP / Foodstamps when I use coupons?

My family is struggling to make ends meet because my husbands hours got cut at work & I have still not been called for an interview, although I have been putting in at least 1 application everyday. We are receiving food stamp (or SNAP) benefits, but it never seems to be enough get through til the next time our benefits are reloaded. SO, I am trying coupons to help stretch those "dollars". I am curious... why is it that when I use coupons, I have to pay the tax for that item? When I pay with food stamps, the taxes are forgiven, but when I use coupons, then some stores make you pay for the tax with cash. I do not understand how this makes sense. Can someone help me understand please?