When it happened to me I noticed that when the pipes were cold the plastic chipped off easy as pie. Not that I've chipped pie off my pipes in the past. If you see what I mean.
Happened to me on my Triumph Speed Triple, once. Found it easiest to let it go cold and then peel off with the aid of a sharp knife. Took about half an hour.
<shrug>
You should slow down for plastic bag when they cross the road and not run them over and you would not have to clean the down pipes .....
Let it burn ride with a peg on your nose