Theo Valich wrote an article over at VR Zone to discuss the origins and implications of NVIDIA's Green Light Program, which prevents add-in board partners from shipping GTX 600 series boards with unsanctioned voltage modifications. The article speculates it's all related to the limited supply of 28nm chips, as NVIDIA probably does not want to share more Kepler GPUs than they have to because the company can sell them for much more money in the Quadro and Tesla lineups:
Unfortunately for AIC and gaming/hardware enthusiast ecosystem, selling a GTX 680 even for $800 per board (which usually retails for $499) - Nvidia knows they need each and every Fermi and Kepler die for their Tesla x20xx and K10 boards, as they're going as hotcakes in the professional segment. We recently learned of Amazon acquiring over 10,000 Tesla K10 boards at a price of just $1500-1800 each, with a mandatory $500/board annual subscription for overnight replacement. Thus, GK104 dies are in very hot demand and with the 28nm yields being the way they are (read: poor) - Nvidia will not do anything to support enthusiasts and damage the premium sales.
After all, it is the success of Tesla and Quadro lineups that enabled prices of GeForce boards to be very affordable. GeForce cards of today feature up to seven billion transistors on a single board (GTX 690) for less money than you'd need to spend on an Intel Core i7-3960X, for example - which offers significantly less compute performance. For price parity comparison, pairing the $999 i7-3960X with a $100 graphics card will result in significantly weaker experience than if you would pair a $999 GTX 690 board with a $100 CPU.