The reason is simply. The experience that Starbucks created to get people to use their stores as a hangout place or mobile office has now permeated through society and has permanently been etched our social psyche.
As a patron, I'm a bit ticked off about this but I also understand the business aspect of this. And this is the subject of this NBC post. One thing mentioned in the post is the wasted space where one guy has a laptop on a small table while the other seat is open but no one will sit there because who wants to sit with a stranger?
Well, I would and I have. I can understand if I go into a cafe with a friend and there are only a bunch of single seats left. Perhaps, a new social norm could be created.
If I go into a Starbucks or a coffee shop to do work and I see open tables, I should not immediately thank my lucky stars but instead go with with someone else who has a laptop occupying a table. But the best answer is probably a time-limited access to the Wi-Fi. Order something, it gets you an hour of use. The post mentioned three hours but I think that might be pushing it.
Okay, maybe two hours. Or maybe the more you spend, the more Wi-Fi time you're allotted.
The bottom-line is let's be courteous of others and business operators, right?