I suspect its more basic than that. I think its to drive up land values.
The Government (read as: Taxpayers) now owns a bunch of land through Fanny/Freddie which is worth far less than the debt taken on when the owners defaulted - So, by taking a bunch of land out of circulation, the value of what remains increases.
And the administration can show its base how environmentally aware it is at the same time - a win-win for them.