Found out about this yesterday.
I personally think piracy is a red herring reason, yes technet is the source of ebay keys but this has gone on for years. I think the real secret reason is microsoft blew their revenues on allowing these new standard subs, realised the mistake and are doing this to force people to buy multiple subs to get revenue back.
They will be very aware people resell technet keys but the people buying these keys wont go to the shop to buy a retail copy if the keys stop been available they will simply instead use a cracked copy of windows that costs them nothing, so 'some' revenue is better than no revenue.
Previously I had only claimed a few keys but following this change I have just harvested about 60 or so keys on my grace period subscription, and am now debating to myself to renew or not as microsoft has blew their trust by doing this on current customers. I think this is what has upset people, even tho the keys may still be enough and be good value is more the fact microsoft changed a contract mid term.