Though I completely agree that if a place is plumb full and you've been sitting their for four hours, it's only polite to offer your table to someone else.
I'm not sure that's a 100% logical conclusion. Would one interpret that to mean that the ~50 people who spend all day sitting in the chairs generates more revenue than the many hundreds of take out customers that pass through each day? Sure, perhaps they make more revenue per individual person, but if you look at revenue per customer type per unit of time, the sit down in store customers are by far the worst revenue source.
I don't deny there's a reason they choose to have attractive interiors, but I doubt it is to encourage individuals to spend an entire day there, so Starbucks can reap the windfall of selling the same customer breakfast AND lunch.
Also, the sitting customers can be segmented as well: most of them will sit down for 15-30 minutes and leave, but there are some who will stay longer. It would be difficult for someway staying 4 hours to spend as much as 8 people staying 30 minutes each, so that's why djt talks about the need to turn over tables.