it does spawn... the mag and any other speedloaders besides m4 mags are quite rare to find. just keep looking in military buildings.
my guess is you didnt play enough to find some in NEAF, NWAF, or BAF.

maybe the character is afraid he might get a cut while the bayonet is attached haha

Katanas are in so many zombie games. Plus i like how each weapon like firefighters axe spawn in places where they might have in real life like a fire station. I dont think the army or residents keep a katana with them.