OP is correct.
this is not about equipping for riot control, or arrests. Not about tear gas or handcuffs. It's about the officer's ability to protect himself. Departments all over the country have determined it is best for their officers to have standard capacity mags.
It's true, an officer is called to the scene of a crime, as part of his job. But if a citizen needs a weapon, that means he IS the scene of the crime.
and yes, they are the same bad guys. And no doubt the bad guys we face will have managed to find one of the tens of millions of "high capacity" magazines now in circulation.
I say any LEO or politician advocating partial disarmament of citizens be required to announce that they have similarly disarmed themselves, or their security detail. Mr. Mayor, if you have three security guards, and you want to limit your citizens to 10 rounds, I say you announce that each of your guards will download to three rounds.