iPhones have had the ability to be remote wiped for a long time. Yet I have not heard of a pandemic of hacker-led mass bricking of iPhones. Dirty hipsters and their iPhones have been at the center of a lot of protests yet we haven't heard of mass iPhone shutdowns by the police in response to demonstrations.
I think government/law enforcement already have the powers they physically need to fuck with cell phones. Between Stingray devices and the ability to present national security letters to carriers or service providers, if they wanted to they could get IMEIs blacklisted or get someone like Apple to brick a specific phone.
It is much more useful to have the phone active and record all conversations. Why would you cut off your "bug?" Also, hacking a phone to brick it is pretty boring. The person is inconvenienced so much that they waste an hour getting a new phone. Also also, anyone with any common sense in a protest is going to use a burner phone, which is much harder to back track to the buyer.