Because some items don't say that they are No Drop when they actually are. That, and sometimes certain quests can only be done by certain classes.
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You know you've been playing too many MMOs when:
You pass a cop and wonder if it 'Agroed' you. Another car passes you, you attempt to /follow it. You use your "push to talk" key while talking on the phone. Text-based games You put a /s before every message you type in an IM (or equivalent / command) (I have done all)
They're not no drop, but something can be no drop and still be turned into an NPC. You just can't give no drops to PCs. Not sure why these aren't no drop actually.