I have my answer!  After scouring the web, talking to anyone who would listen at the pet store, and pleading with the vet for years.  Today Samantha said, "I got it.  I know why the cats eat plastic bags."  She had been rereading some old National Geographic Kids magazines and came across a question and answer page.

"Plastic bags are made of a substance that comes from petroleum, and the scent and texture drives many cats wild."  Of course they are not safe and despite the picture we do not 'let' them play with bags.

I now have peace...  Well, maybe not.  But, it still feels good to know that our cats are not as self-injurious as I once thought.