Even a DFU restore of the device won't help a thief, as the activation process will simply ask for your iCloud login and will display a "Message From Owner" that you can set at icloud.com indicating the device was stolen, making it much harder for someone to purchase and claim ignorance about the origins.
This way if you're on the subway you can have your phone out without worrying about getting robbed.
It your house gets robbed they would leave your phones because they can't be sold or reused.
You might not buy a phone from an internet cafe that sells 2nd hand phones, but you might buy a replacement screen from an internet cafe that does repairs.
Short of visibly, physically destroying the phone, though, there's nothing you can do to prevent this. A criminal doesn't care that the phone is a brick; they'll sell it to someone and be miles away with the money before the buyer realizes the phone is useless.
Then again, this might create lots of other problems.
The phone won't do anything until the person who turned on the lock turns it off again.
Why would I want to give a third party the ability to brick my phone? If Apple has that power, then a disgruntled Apple employee can use that power and a government can compel Apple to use that power.
My computing devices are my computing devices. Only I should have the ability to do anything to them.
Apple need to create a system where stolen phones can be reported to them, Apple can then contact the owner/verify they are stolen. And then add them to a stolen list and disable calling/apps on those phones. And display an overlay on the screen THIS PHONE IS STOLEN.
Every iphone would come with an validate phone feature that is accessible even when locked that can authenticate the iPhone for anyone thinking of buying it.
The potential buyer can check if the iPhone is stolen by using the feature that is allowed to connect to the internet and validate the phone.
They need to make it where stolen iPhones are worthless so when you are getting mugged criminals won't even want it.
Obviously have an option setup where you can transfer ownership of your phone. Maybe with a 7 day waiting period.