The thing that makes them legal is the magazine release - notice the funky-looking mag release in those pictures I posted. It allows you to easily release the magazines, but not with your finger - you must use a tool, like a bullet or other small object.
From CA PC 12276.1:
From CA PC 12276.1:
“detachable magazine” means any ammunition feeding device that can be
removed readily from the firearm with neither disassembly of the firearm action
nor use of a tool being required. A bullet or ammunition cartridge is
considered a tool.