Consider:
>unarmed protestors
>worst case scenario happens
>they get angry and riot
>break some windows, destroy some cars, maybe torch a few buildings
>cops, firmly in control with riot tactics, clean it up with no serious threat to their lives
The cops decide when these protests end and they do not hesitate to stop them shortly after shit starts to look ugly.
>armed protestors
>worst case scenario happens
>they get angry and start shooting
>cops aren't in control, need to call backup and eventually overwhelm them
>lots and lots of casualties on both sides
The cops eventually will overcome but only at great cost. A firefight is the absolute last thing they want.
Can you see why they pounced on the "ringleaders" as soon as possible, even if they have to martyr one guy? Their strategy is based on the worst case scenario, and that worst case scenario is a violent flashpoint that starts new militia movements like it's the 90's. A bunch of dindus looting stores won't start anything more than white flight out of the inner city so cops don't really give a shit.
Conclusion: They couldn't defuse the situation, so they cut the dynamite in half and hope the fuse fizzles out.