Why?

From a Sacramento Bee editorial (with emphasis added):

The [federal transportation] bill expands tax breaks for all hybrid cars, giving hybrid buyers not just tax deductions, as previous laws did, but straight, dollar-for-dollar tax credits of up to $3,000.

Under the bill, the higher the fuel efficiency, the higher the credit. That's not a bad thing - except that the tax credit is based on weight class. So buyers of gas-guzzling hybrids, a big SUV or pickup that uses some hybrid technology, but gets as little as 15.1 miles per gallon, can still qualify for a federal tax credit. That's absurd.

The law also apportions the credits among car manufacturers, allowing for a tax break on only the first 60,000 hybrids sold. So by 2007, buyers of the fast-selling Toyota Prius hybrid that gets 60 mpg in the city won't receive any tax credits at all, while buyers of GM's Chevy Silverado hybrid pickup truck, which gets only 17 mpg, would qualify for more than $1,000 in federal tax breaks. That's no path to energy independence.

And then, as the editorial discusses, the bill endorses single-occupant hybrids' use of carpool lanes. Buyers of high-mileage hybrids deserve incentives, but not that.