I think the "addictive" ingredient is monosodium glutamate. It makes your taste buds go all "jangly". Same powdery stuff that is often added to chinese takeaway food.
There was a documentry (Horizon if memory serves) on an E-number which can be found in, amongst other products, Pringles. It is slightly addictive but nowhere near enough to be illegal. But some people maintain that this particular E-number shouldn't be any products at all. I'm not even sure if the substance is even still put into Pringles.