The problem is not strictly the pennies themselves, but all of the prices that rely on being able to quantize things to a cent, and a number of different laws about not playing games with prices.
Most recently, a stick in the SNAP benefits laws is that you can't charge SNAP recipients different amounts from other people - which was presumably intended to ensure you can't play games like charging SNAP recipients more for things, but in practice, means that if you, hypothetically, wanted to charge SNAP and credit card holders exact amounts (which you would likely want to do to avoid weird effects where SNAP recipients, who tend to be very price sensitive, find their bills going up), and charge cash users rounded up or down, you would be in violation.
Those are the kinds of warts you would hope to see a plan for before these things were announced, rather than having to figure out one in the middle.
Most recently, a stick in the SNAP benefits laws is that you can't charge SNAP recipients different amounts from other people - which was presumably intended to ensure you can't play games like charging SNAP recipients more for things, but in practice, means that if you, hypothetically, wanted to charge SNAP and credit card holders exact amounts (which you would likely want to do to avoid weird effects where SNAP recipients, who tend to be very price sensitive, find their bills going up), and charge cash users rounded up or down, you would be in violation.
Those are the kinds of warts you would hope to see a plan for before these things were announced, rather than having to figure out one in the middle.