This is a question of a fashion which started in the USA where self conscious/insecure people abound (self-help suggestions making lots of money) - it matters to ostentatiously carry something that is assumed to be a proprietary item and to have cost money - the aim is to be part of what used to be "the in crowd", later simply being "cool". Quite apart from the waste/mess created by the vessels used, most of the time during which it is carried the tea/coffee is way too cold to be enjoyed as a drink, if that ever was the point in the first place (the cups are very often disposed of part full, often parked somewhere once the owner's hand/arm becomes too tired). That said, as absurd as the practice is, it is not realistic to force people into giving the habit up (not that the OP suggested it), one can only hope that the people concerned grow out of it. I have long intended to make a counter statement by carrying around an open tin of sardines but, like many other plans, this one is still pending execution.