This was a new one for me, I had never heard this one before. Of course, I don't have cats but we did when I was a kid and our cats never chewed plastic and didn't seem to have any desire to either.
Interesting though, since plastics can be made with a variety of products, one of which is by-products from the rendering process. Therefore, plastic bags may contain casein, renin, or other animal by-product that both smells and tastes appealing to cats.
Cat owners should not allow this behavior since their cat may swallow the plastic, causing irritation to the stomach and intestines.