Oh, no, TV Tropes. You've just doomed people to hours of interesting. (Knows what Fridge Logic is and refuses to click! Refuses to click!)
I figure that the Deku Shield is very tough. There are kinds of wood that can be very, very tough (so much so that dentures were made of some types of wood in the old days). Ever hear of ironwood? Back where I used to live, it is a scraggly desert tree - very dense wood, so much so that those that carved it created scultptures that looked like they were made from polished stone.
Also, it is sacred wood, taken from the Great Deku Tree (a deity, guardian of the forest, barrier guardian of Hyrule).
Yet, it goes up like newspaper when touched by fire. Everything must have it's advantages and disadvantages, paritcularly the "weaker" items in a video game.
Of course, there could alwyas be a "Mythbusters: Hyrule Edition" to deal with all these fridge logic things. In an unrelated legend, they actually *CONFIRMED* that bomb-arrows would/do indeed work!!!!