Mtbe was basically toxic waste, a byproduct of the oil refining process. Today, all federal reformulated gasoline contains some MTBE, but not as much as the CA version.
The whole deal is a scam. MTBE is a carcinogen, and is finding its way into ground water, and even Lake Tahoe. The oil companies have benefitted by charging more for MTBE gas, and not having expensive toxic waste disposal.