Sounds to me like you shouldn't have taken what she said at face value.
Not really. With $X.99 prices, you're being rounded up if you buy one or two items, and down if you buy three or four. The absolute most you'll ever miss out on is two cents per cash transaction, and that's roughly balanced out by all the times you make an extra two cents. Using the magic of Excel, I just took a look at my debit/credit statements up to the end of July (I haven't done August yet) and rounded all of those numbers to the nearest $0.05, and apparently I would have lost 28 cents if I'd used cash for every single one of those transactions (which is unrealistic, especially where some of them were online bill payments). I'm okay with missing an absolute maximum of 28 cents in seven months. That's not particularly significant on that scale.
If thats an average, then $0.28 x 37.6M people = $10,276,000 per month to businesses. And to clarify for others, when paying with cash, after taxes, prices ending in $X.X3 round up, and prices ending in $X.X2 round down. So people will use their debit card when its three cents or more to save those extra couple of pennies.
Its good to know they dont have any obligation to take our money, however. Which would be strange if they refused in a normal situation.