Approaching this from a different angle, if the staff of Star(b*)ucks have issues with intending passengers using the facilities without purchasing anything, what is to say that the passenger does not want anything or having had a look at what is on offer, they do not like what is on the menu? * Insert consonant of choice.
Or how about that they may have previously purchased something, but do not or do not like what is offered that day?
Furthermore, what nobody has seemed to have mentioned yet in relation to it being a business, it is not just an ordinary run of the mill business. It is a large corporate business that does everything possible to get out of paying tax in the UK to the UK government. If it was an individual from a working class background who tried that stunt, it would be highly likely that they would become a house guest of Lizzie Windsor in jail. Perhaps these are the sort of questions that should be raised?