It's the nearest choice most place have to what you suggest. There are few venues that allow you to pay directly for table time, but you're (ehtically) doing the same thing by regularly purchasing items from them.
I guess my point is, that its probably about time we had different pricing models. I am not trying to be obtuse, and i do get what you are saying, but regularly purchasing items is (in my estimation) a bigger problem than the one it solves.
obesity+consumerism > slightly broken cafe market
I would suggest a nominal rate (after the first hour or two) would make everyone happy. It is quite likely the average stay would increase as people would not have taking up space on their conscience.
