Once Amazon knows it was stolen, they do everything they can to ensure it is valueless to the thief.
But they can't or won't remotely brick it.
...
They have blacklisted it, which prevents it from being registered and thereby disables a lot of functionality but still allows the thief to
sideload books and read them without collections support.
It isn't worthless, just crippled -- and since
most Kindle thieves don't actually want an ereader, they want the money from selling an electronic device, they have lost that opportunity.
Smart Craigslist/Ebay buyers will make sure the device can be registered before handing over money (Craigslist) -- or report it as a fraudulent purchase and demand their money be refunded (Ebay).