A good question from Trent McBride at Distributed Republic:

When Republicans use free market rhetoric while campaigning, only to reverse when in office, we (most libertarians) decry their hypocrisy and warn not to believe future statements. And likely correctly so.

However, when Democrats spout socialist rhetoric, I would argue most libertarians take it as face value and fear they are speaking their true feelings, unlikely to stab their ideological constituents in the back. Why does this asymmetry exist?

This sounds like a classic case of pessimistic bias to me!