https://www.ecfr.gov/current/title-7/subtitle-B/chapter-II/s...
"Coupons shall be accepted for eligible foods at the same prices and on the same terms and conditions applicable to cash purchases of the same foods at the same store except that tax shall not be charged on eligible foods purchased with coupons."
The law did not account for every possible situation. Removal of the penny from national currency is clear a situation where minor variations on otherwise normal transactions would not be in violation of the intent of the law.
It'd be like TSA griping that your 100ml bottle of mouthwash was overfilled by .1ml because of slight variations in the filling process. Nobody cares.
Last week, the government was in a shutdown and it was unclear if SNAP benefits were going to go out. That's not the same as rounding pennies.
Presumably that's fine because a SNAP recipient has access to those same discounts. So wouldn't this be the same - the "cash rounding" discount is available to SNAP and people paying cash?