A netiquette for coffee shops, I wholeheartedly agree.
I work at coffee shops a lot; I keep sitting there when I can see there are plenty of empty tables - that means my sitting there is a net positive for the Cafe: I might consume more, and often do. And having customers is better than having none, it attracts other people.
If I see that many customers are coming and that I might actually block people from getting a table, I leave.
Only exceptions is Starbucks because its a huge chain and they have always had an official sit as long as you want policy which has worked out very well for them. I am not sure about the actual business trade off but I often end up at Starbucks because I know I can sit there as long as I want - this business might otherwise have gone to independents. I realize though that what works for a large chain might not work for the coffee shop on the corner.