It's just such a bizarre choice that one might hope there would be another interpretation. Why measure a percentage change on sales tax, which varies heavily from location to location, and is not what the associated fees are based on, rather than simple choice of total cost?

Because sales tax is something you pay that's more than the sticker price, and people tend to have an intuition for sales tax in their area. Personally, I find "a 3% credit card fee is like paying 21% more in sales tax" to be intuitive.