Since the Camp is a direct blowback action, the bolt starts moving (and bringing the case along with it) as soon as the cartridge is fired.
Handguns that shoot 9mm and greater use a 'locked' action that generally moves the barrel along WITH the slide for a short distance before the barrel 'drops down' unlocking. The slide then continues on it's merry way bringing the casing with it.
The pressures generated by 9mm+ cartridges will rupture (expand it past it's elasticity point to it's breaking point) the casing if it wasn't contained by the steel chamber. The 'locking' action of handgun barrels ensures that during the time of maximum pressure, the cartridge is safely contained.
In a blowback action rifle, that just isn't so. To delay the rearward movement of the non-locking bolt, they design the bolt heavier to give it more inertia. By adding a heavier recoil spring, you also add a few milliseconds delay to the bolt moving rearward while the cartridge is generating high pressure. If the casing leaves the protection of the chamber while extreme presssure is being generated, someone will most likely be eating hot gas and brass shrapnel.
I would NOT use +P+ (+pressure+) cartridges in a Camp 9 with stock springs. The manual is absolutely correct there. I'd do +P with the 16.5# and would consider +P+ with a 21# spring, but that would definately be at the owner's own risk.