Put red hot chili sauce on some bags.
There is something corn starchy (hopefully not talc) they put in the bags to keep them from sticking together, that is mighty tasty.
I'm not sure I can offer you any help, but I can relate. Our inside cat (we also have 4 outside cats) does the same exact thing. She will lick plastic storage boxes, plastics bags, garbage cans, etc. Hopefully someone else will come along that can answer this question because I too am curious as to why some cats do this.