The simple answer is that politicians want it to happen — no matter what the economists say. (The fact that some economists also support the PC is practically irrelevant; notice how Schwarzenegger is pushing for the PC without waiting for any scientific or economic justification.)

This case reminds me of when politicians ignored economists in making the decision to build the State Water Project (which was called the Feather River Project at the time). Read this 1957 paper [PDF] to see what the economists said.

They also ignored economists economics when they decided to build the Colorado River Aqueduct in the early 1930s. See Sections 3.2 and 3.3 of my dissertation for more on that.

Bottom Line: Politicians like to promise everything to everybody, but somebody has to pay — and sometimes we ALL have to pay.