AFAIK, it's deliberate. Alien and alien-derived lasers are powered by Elerium. Or, more accurately, are using Elerium as an energy storage medium.
IMO, making an extra item will do nothing useful, since you still have to disassemble and then manufacture the PLC ammo. And 'realism' would say that 'laser rifle clip parts' are not the same as 'laser cannon clip parts', especially since one is of X-Com manufacture and the other is made by shadowy orgs or perhaps even the aliens themselves. So there'd be another extra step.
One could, of course, make a submod that directly converts Elerium-based clips of one type to another, similar to the streamlined armour-manufacturing submods of Piratez.