Nothing but pure curiosity and am in no way insinuating your opinion is wrong, but have you ever waited tables? I say this only because I feel that it is difficult work and being that waiting tables has little similarity to cooking at home. Nothing in the kitchen, at least in mine, compares to the arrogance and brazenness a waiter will receive in a given shift. From 6 years of experience, I will say that 99% of the difficulty faced in this position is not from the actual order taking aspect, but instead a product of extremely fast paced working environment, mixed with assorted rude souls. Being on your feet for 8 straight hours waiting hand and foot on people who show you little to no respect may change your mind.
Dramatics aside, it can also be a very fun working environment and I wouldn't trade the things I learned in that profession for anything. And for those who are mentioning it... the 3.63/hour wage gets completely taxed out by the claiming of tips. I don't think I ever received a paycheck when I waited tables. But do not let that fool you as any half decent server makes well over minimum wage, even with the bad tippers. I also believe that this store owner is completely out of line. Though frustrating, nothing states that a tip is required, and in no way do I think the public shaming is validated.
P.S. I hope that you giver her more than 20%