I believe that requirement only applies to 2 restaurants in Down Town Disney. I am not sure which 2, (I think Wolf Gang Puck and the Rainforest Cafe) but it is clearly delineated on the official Dining Plan Brochure on the official Disney Web Site.
Other restaurants do not charge a surcharge for regular menu items. There are some restaurants with an "Add-On" available at an extra charge. An example of an "Add-On" would be a lobster tail added to a filet for example. The filet would be the menu plan item, and the lobster tail the "Add-On". This policy applies to everyone, whether you are on the Dining Plan or you are paying OOP.
HTH