I've wondered why there's such a line at the Manual booths, when the credit card lanes are totally open. Most people here have credit or bank cards, so why pay cash? It turns out that this is quite closely connected to Spain's wonderful underground economy (Woohoo 2nd in Europe after Italy).
As a salaried employee, you cannot escape taxes. Everything is done automatically for you and the deductions are few and far between.
However, running your own business gives you almost limitless opportunities to cheat on your taxes. One of the challenges in any semi-legit business is how to recycle your black money into clean tax deductions.
Thus, paying tolls with cash gives you a double benefits: use the black money for something useful, while saving the receipt to get the tax deduction that amount.
Okay all you salaried mileuristas, go back to your jobs, nothing to see here. Suckers.