Well...
1.) They have to now pay for all the highway repairs, that they aren't generating revenues from with greener vehicles.
When your Prious gets 50 mpg, you simply don't contribute a whole lot to road repairs.
2.) Obama is still on his green energy kick, trying to make Gasoline unaffordable, so green vehicles that contribute less to road repairs are more popular, so that we have to pay more taxes on gas, so we can sell more green vehicles.
If I had to guess, somewhere along the way, there are a whole lot of subsidies going into the green vehicle technology game, that are being paid for by gas taxes.
3.) Obama, who said, he was going to cut taxes and reduce the deficit 7 - 8 years ago, when he was running for office, has run into the reality that the deficit has been turbo charged and is running away under his expert tutelage, and he's just trying to fund some of the [beeep] he doesn't have money to fund. Because of course... We didn't build any of that.