Believe it or not, people ask me these sorts of questions almost every day in my practice. So, to answer one of the questions, yes, a very large number of cats like to chew on plastic bags. Many cats also enjoy chewing other forms of thin plastic, or chew on (and sometimes swallow) rubber bands.
I do not have a definitive answer for why cats do this. It may be because they enjoy the texture, which is unlike most objects found in nature. But, frankly, that is just a guess. Certainly, medical science has provided no answer to this question that I’m aware of.
If your cat is not swallowing portions of the bags, then it probably is harmless. The plastic in most bags is inert and not toxic. Even if swallowed, small pieces of plastic (or rubber bands) usually will pass harmlessly through a cat’s system. However, a slight risk exists that such foreign material might coalesce into a form that can obstruct the intestines and require surgery. As well, if any toxic materials are on a bag (for instance, if the bag was used to carry insecticide or motor oil), they could make your cat sick. And, a risk of suffocation exists if cats crawl into plastic bags. So, to be safe, you should probably keep plastic bags out of reach.
However, if your cat is like most, she will probably continue to chew on the occasional plastic bag without coming to any harm.