This started with touch-tone dialing back in the 70s.
Remember that, in those days, every phone charge was set by the various government regulatory agencies.
The touch-tone tariff was just a continuation of the "soak the rich" philosophy exemplified by high rates for business phones, modem lines, long distance, and other services to subsidize the cost of unmetered local POTS for the masses. Tone dialing was seen as a "luxury" feature that could be taxed to spare poor and lower-middle-class folks the capital costs of upgrading aging central office equipment.