The powder, including the black powder subs falls under the explosives act.
The camp fuel etc simply comes under the "dangerous goods" act and there is all kinds of non-sense regulations that are different depending on how much is shipped (so you see cases that say things like "limited quantity" or "small package", which is a low enough quantity that the more severe reg's don't kick in).
Boat flares, auto flares and fireworks come in the same category/regulations as powder and they "ARE" handled, stored and transported differently at "additional" shipping costs.
So while it is true that powder incurres additional costs, pretty much the same as if you order your powder on-line, it's by no means a "show stopper" if the store WANTS to carry it - after all, those additional costs are reflected in the price on the shelf.
What is true however, is whether I order an entire pallet of powder or 10 pounds, the DG costs are relatively consistent. So I doesn't make sense to order in a "few pounds" at a time.
But then you start getting into another area under the explosives act whereby if I "warehouse" above a certain quantity then I have to have a dedicated "magazine" (and sorry, don't remember all the different levels and compliance there) - so in many cases it's just not "viable" to carry a small quantity (the price would be too high on the shelf) but to get the cost down the red tape and storage requirements go way up.
Not that the government is trying to make it difficult or anything...