It may be non-traditional, but I have seen these things advertised long before inlines became popular. There are also versions for small rifle primers. These were developed when Pyrodex became popular. Because it has an ignition temp of around 600 degrees, it really needs a hotter ignition than BP. Many people use Pyrodex because of the lack of availability of BP, so things like HotShot nipples, magnum #11 primers, and adapters like the one being discussed in this thread were developed. I, for one, don't have a problem with helping someone out who is making a concerted effort to hunt with a sidelock muzzleloader. If we are all forced to use subs because of draconian laws regarding BP, we may all have to look for alternatives for better ignition. I would much rather see someone choose this device over a modern rifle that happens to be loaded from the muzzle. I am all for enforcing the rules, but it seems to me that this type of device doesn't deviate any more than using coil spring locks, man-made flints, fiber-optic sights, etc.. If I am way off base here, I am sure I will be swiftly and soundly admonished.
Larry