Finally, I know. Today, a friendly ACS employee told me the whole story.
When you sell a car in California, the seller is required to file form REG 138 within 5 days, and you can actually complete it online here. You enter the license plate number, the seller's name and address, and the buyer's name and address. The state then considers the buyer responsible for any traffic violations. The buyer is required to take the title to DMV within 10 days to complete the transfer of ownership.
Toll violations originally were sent to the registered owner. But then, the registered owner put my name as the buyer on a REG 138 and gave it to FasTrak, who never checked to see if I really was the buyer. They just started sending me the bills for toll violations. How convenient for the registered owner ...
I don't even know if the REG 138 was ever filed with DMV. They don't notify the person listed as the buyer when liability for a vehicle was released to them. So, the system has no barriers or checks to prevent fraud. Anyone with an internet connection, anywhere on earth, can file REG 138 online and enter fake information.
What a disaster!