That used to be the case, but it certainly isn't anymore. 95% of gas stations in Los Angeles have a separate, lower, price per gallon for cash.
The federal ban on charging fees was lifted, but many states (including New York and California) also still outlaw this at the state level.
In addition, merchant agreements often prohibit it - for example, merchants that take American Express cannot charge fees for any transactions, even the ones that run on Visa or Mastercard.