They do have a sub air system in that green, but thats not what is keeping the snow off the turf. There is a heated coil system that allows the grass to grow year round-it can get pretty tough in that corner.
Tony, thank you for that correction. Yes, the hillside and the surrounding trees keep 12 green in shade, and I have always understood that the grass there tends to be thinner and more prone to spike damage than anywhere else on the golf course. 12 was the very first Sub-Air green on the course, wasn't it?
I do recall that 12 was where Vijay Singh lost his temper with Phil Mickelson's extra-long metal spikes, playing the group in front of him and leading to the near-fistfight in the the Masters' Locker Room. It is also where, in 1986, a spike-mark knocked eventual-winner Jack Nicklaus' putt off line, promptiong Jack to jab at it with his putter.