IT IS SIMPLE, we need high cap mags and ammo because we are subject to the same dangers as cops and the military.
I actually think the reverse of this. Bear with me here. I hear people often say "well, that sort of magazine capacity should only be held by police or military." I argue that the police could get away with 10 round magazines while I can't. My arguments, in no particular order:
1. Criminals avoid the police.
For me: Criminals look for victims among the civilian population. I'm a civilian and I don't look physically imposing at all.
2. The police usually choose condition of the engagement. Patrol cars choose the time/place of their traffic stops. Or they receive a radio call and they know that they are going to dangerous situation such as a robbery in progress. They already know if other police are responding to that same call and how far away they might be. They prefer to outnumber the bad guys and they can wait for help to arrive. They can call for backup before proceeding in many circumstances if they don't feel comfortable.
For me: if I've met a criminal, things have already gone completely wrong. I don't do any criminal type activities and if I meet a criminal or a band of bad guys, they have probably the initiative since they have picked the time/place of our encounter. I'm a step behind and probably outnumbered.
3. Police have radio and active backup already mobilized. If the patrol officer feels endangered, a call for backup will bring all available help immediately.
For me: i may or may not have time to call 911. I might be immobilized. I might have to make an immediate decision to put my life in danger against superior numbers to save a family member.
So, for me, a normal capacity magazine (usually more than 10) just makes sense for citizens who are quite probably in an inferior tactical position in a criminal encounter. For police officers, they might not need that additional magazine capacity as they have several levels of tactical advantage and support.
I'm not trying to downplay the dangerous nature of police work nor saying that I don't appreciate that they do a job that I can't (see the bit above about not physically imposing). Nor am I saying that police should be limited to 10 round magazines. I'm just stating how encounters between me (the average joe) and a criminal/s appear fundamentally different from a tactical perspective than for a police officer/s and a criminal/s.