Tipping - the American Approach

I was interested to read about an American woman who was literally hounded and harassed on-line for not leaving a tip in an Atlanta, Georgia restaurant. She says she did leave a tip but admits to writing an online review in which she said the food was "tepid". In retaliation, the restaurant manager put a photo of her on Facebook and Twitter, and warned other restaurants owners not to let her into their establishments. Oh, and he said they should all tell her to "Go F**K herself". 
And he wonders why he didn't get tipped?
The owner of the restaurant has since apologized and offered free meals etc to make up for things, but it's the comments of the readers that interest me. Although many of them didn't quite go as far as defending the manager, they roundly criticized the habit of not leaving a tip, even when presented with tepid food, as in this case.
I've lived in the States for over 20 years, and no matter how bad a meal or service gets, I have never met an American who is comfortable with leaving no tip at all. The reason usually given is that not only do wait staff earn a ridiculously low hourly rate (and therefore need those tips), but the problem is not always their fault. In my experience however, even when the problem is directly because of poor wait service, a tip of some size will be left.  Given that the tipping rate here is not less than 15% and usually closer to 20%, a reduced tip to reflect reduced satisfaction is still better than a slap round the chops with a wet lettuce. 
Such is the culture of tipping in this country that you can buy tipping ready-reckoners, the size of credit cards, to help you calculate the required tip. For the more tech-savvy, there are countless tip calculator apps and web sites, such as this one, that not only figure out your tip, but tell you how much each person should pay if you're splitting the bill between a number of people. 
It's much harder to come across advice for tipping when the service is bad, and one site even pulled up this comment as proof of my point:  "If we eat a meal with absolutely horrible service, believe it or not, we still tip. However, we NEVER go back again and let everyone we know of the bad service. I think that's even worse. If you own a restaurant, you want feedback. If people all of a sudden stop showing up, I think that's much worse than giving a bad tip."  
Well yes, bad word of mouth is a killer, but you STILL TIP. Isn't that a tad cowardly? Although given the lead story here, you never know what's going to happen these days if you skip on a tip.