Do you really not understand why the waitress's demeanor changed the moment you mentioned using a coupon? Seriously?
You know wait staff live on tips right? You know that people who use coupons are notoriously bad tippers right? There are very, very few people who feel they got a deal so leave a bigger tip. Most leave a tip based on what they paid, not on what the meal should have cost and people that use coupons tip at the bottom of the scale to begin with.
If this is the case, then as far as my reading of the story goes, upon discovering that she was serving people who she believed would be stingy tippers, she behaved in a way that would make even a generous tipper leave nothing.
You know wait staff live on tips right? You know that people who use coupons are notoriously bad tippers right? There are very, very few people who feel they got a deal so leave a bigger tip. Most leave a tip based on what they paid, not on what the meal should have cost and people that use coupons tip at the bottom of the scale to begin with.