Yes, this is a real problem some places. At the closest Starbucks to me, there is practically never a free seat because of all the people camped out for hours at a time every single day. That means people with more reasonable needs, such as a place to sit for a few minutes waiting for a friend to arrive or complete an errand nearby, are out of luck. Those few are degrading the experience for many more, quite likely costing the store more than their own paltry consumption could justify (especially in a market with many alternatives).

Of course, this is exactly what anyone familiar with the tragedy of the commons would have expected. Self-regulation is not nearly as common or as effective as many would have us believe. Those squatters could yield this quasi-public space to others and work from home. They choose not to, knowing full well that they are being selfish by over-consuming that shared resource. They probably even know that the free lunch will not last forever. Nonetheless they persist. The proprietors have no choice but to take matters into their own hands, protecting their own interest by setting and enforcing limits.

With the right hardware it would be easy to tell which MAC address corresponds to which seat, and of course the proprietor would know when the occupant last purchased anything. That information should be sufficient to do the right thing when seats run out. ;)