At the San Mateo County Fair, parking is $15 with cc or $20 in cash. This may be due to risk of robbery, or because they have no way of reconciling their receipts against any sort of inventory (and do not trust the employees to turn over all of the cash).

I have to think robbery isn't the main concern, however, since the lines are typically so long it would be hard to get in and out rapidly (and there are a decent number of cops around).