I understand the office space argument and can see perfectly well how coffee shop workers should not be required to distinguish between all mobile devices. All perfectly fair.
That said, here in Brazil, carrying tablets in the bus is quite like an invitation for robbery. Which is why some smartasses developed the brilliant idea of carrying their tablets inside hollow books: they carve out a window in the book's pages where their tablets will fit.
So, people thinking they are rightfully entitled to some reading while in a cafe, be it on physical or digital form, might give it a try.