The keys were deactivated because they were being sold on shady websites as discounted game keys. The keys were originally bought on steam with stolen credit card information, and then sold to these websites.
The original credit card owner would initiate a charge-back, getting a refund from Unknown World Entertainment, while also incurring a fee from the credit card company. In total, Unknown World Entertainment loses out on ~$45 (~$22 fee + $25 game price) per key.
What makes this even worse, is that the criminals who stole the credit cards and sold the keys to these websites, gets away with it. I guess the lesson to learn here is that, if it sounds too good to be true, it probably is.
For now, if you’re interested in picking up Natural Selection 2, you should get it from the two official sources; Steam, and Unknown World Entertainment’s official website.