There's a line of thought that any flues, pipes etc. which cannot be guaranteed to be isolated must be securely bonded instead.
The application of one of two opposites sounds kind of screwy at first, but I think the philosophy is to avoid possible arcing at unintentional points of contact in the event of a fault (such as the flue/siding contact you mention).
That said, I've seen one or two wood stoves installed here where even in a regular house bonding the flue might have been a good idea. The installer had run the flue up through the roof space with no regard for existing electrical cables, leaving them in danger of contact and heat damage.
