There are several theories as to why cats may lick plastic. My thoughts are that cats are attracted to things that move, things that are shiny and sparkle – things that can mimic prey.
Some believe that cats like the texture of plastic when they lick. Another theory is that some plastic is made with some rendered animal fat which cats may be able to smell or detect. Another explanation is that some plastics have petroleum products and gelatin as ingredients.
For example, my male cat Hero, LOVES to lick tape. I’ve always found it to be really weird. He also loves to suck on plastic and tape as well. Not only that, but he sucks on wool and nurses it like a baby kitty even though he’s full grown and over 2 years old. So bizarre. Thankfully he doesn’t eat the wool, and only licks the tape until I snatch it away from him. 
Regardless of why they do it, cats will often play, pounce, prey and sometimes eat those objects. It becomes a problem when cats cannot digest it and it becomes a gastrointestinal obstruction.
For cats that have this tendency to eat strange items, it is very important to keep plastics picked up and unavailable to cats. I myself don’t always remember, but I usually am quick to respond to the rustling of any plastic sound. 
For more information about a disorder attached to this behavior – research more on ‘Pica in Cats‘.
“Pica” (pronounced “PIE-kuh”) is the voluntary ingestion of non-edible materials. The term comes from the Latin word “magpie”, because magpies are said to eat almost anything. Pica accounts for approximately 2.5% of abnormal behaviors in the domestic cat. The etiology of true pica is not known, although mineral deficiencies or psychological disturbances are often blamed.
