The point is if you buy a coffee and drink it at a little table in the coffee shop, and then leave, you may have spent 5 to 10 minutes there. But if you're going to set up your laptop and work 2 hours before purchasing another coffee, then you're taking over a space which could have served about 10 more customers in the same time span.
In addition to that, if you're there to work, chances are you're there by yourself. However other customers may come in groups, and that table can be used by the group. By taking over the table and having that group pass the shop for another place where they can drink/eat+talk, you're costing the shop a lot of customers.
In addition to that, if you're there to work, chances are you're there by yourself. However other customers may come in groups, and that table can be used by the group. By taking over the table and having that group pass the shop for another place where they can drink/eat+talk, you're costing the shop a lot of customers.