Regular people understand the decimal system and they understand rounding in that system. The floating point system is a different system that they don't understand. People need to trust that the mechanics of payments are correct or they feel like they're being ripped off, and you can't really trust what you don't understand. Rounding of decimal pennies means that people get payments systems they can trust at the cost of precision. Nobody cares about sometimes being up or sometimes being down a fraction of a penny.