Because they don’t pay any gasoline tax. From the Seattle Times:
Owners of electric cars in Washington state don’t buy gasoline or pay gas taxes, but they’re soon going to be hit with a $100 fee to own the battery-operated cars.
A section of Washington state law that takes effect next year will require electric-car owners to pay a $100 annual fee for road and highway improvements, intended to compensate for the gas taxes they no longer pay.
The law does not apply to hybrid vehicles or to those that don’t exceed 35 mph. About 1,600 cars currently registered in the state would likely be subject to the fee, including the Nissan Leaf, Tesla Roadster and some custom electric vehicles, according to the state licensing department. Hybrid vehicles that use electricity and gasoline, such as the Toyota Prius and Chevrolet Volt, are excluded.
Starting Feb. 1, electric-car owners must pay the fee at the time of their annual vehicle-registration renewal. The fee would be in addition to standard vehicle-registration fees owed each year.”
Just how is it that the Chevy Volt was excluded? According to Wikipedia it has a 1.4 liter four cylinder engine and is classed as a hybrid.