Ocean's Eleven bitcoin edition...
GPU based mining is mostly done to mine Ether. Ether will switch to Proof of Stake somewhere in the next 1-2 years, after which the competition between gamers and miners for GPUs will end.
Also, you're assuming whoever stole these cards are rational beings and not characters out of Ocean's Eleven.
They are cheating by putting it near to a known hash to verify that it works, you can't earn anything off it.