It's not called the far left for nothing.
They take care of their own!

Because hybrid cars use less gas. Same thing happens with you carpool. Maybe they thought it was doing the same thing so why not allow them to use the lane?

But the idea was to lessen pollution by reducing the number of cars on the road. More cars means more congestion, hybrids or not.

The idea is to reduce pollution. By encouraging people to buy hybrids, they are reducing pollution. Of course, there are non-hybrids which get better milage than hybrids, so politics definitely is involved.

The idea was to reduce pollution by reducing the numbers of cars on the roads. Allowing single driver vehicles to clog up the road creates more pollution by creating more congestion.

In most cities, HOV lanes are under utilized. They're wasteful, because they can carry much more capacity than they actually do. It makes sense to save energy by moving some of the cars from the non-HOV lanes, into the HOV lanes. If you're trying to encourage energy efficiency, then it makes sense to move the most efficient cars into the HOV lanes.

They're underutilized because people don't want to car pool. And since this is a free country, we shouldn't be forced to. (Nothing wrong with CP, just don't force me to.) If the lanes aren't being used, then open them up to everybody. Non-hybrid owners pay more in taxes for that lane anyway, so they ought to be able to drive there.

Cities are generally moving away from HOV lanes, and now going to "lexus lanes" where you pay more to use the high speed lanes depending on how bad the traffic is.