Olive Garden's secret recipe is an arcane gate hewn from the bones of long dead eldritch horrors and fed the souls of gibbering demon spawn to fuel a portal into the Oliveverse, where bread stick barrens and seas of salad stretch towards infinity.
Or they just serve shitty bread and a sad little salad with only one shard of tomato and one half olive. That way you don't take them up on the "unlimited" part....
Or they're poisoned by mercury and the rare individual bearing the taste will run out of health before their profit takes a hit.