I know it is. That rule, in itself, is not entirely clear, as there is some sort of upmarket refreshments pod in the middle of that seating, which means that it seems rather like those seats are for paying customers of that establishment. Some people work out the real situation, whereby anyone can sit there, and some don't, unfortunately. I have eaten my lunch up there twice now, and I certainly hadn't purchased it within the station, and of course there was no problem with that.
Unfortunately, not everyone will think to go to the upper area / mezzanine and will think it is for restaurants or platform access for first class passengers only. The escalators are not signed clearly enough and are a little bit tucked away on first glance. Also, if everyone goes upstairs, the number of seats and tables will simply not be adequate. People with lots of luggage or who don't want to walk far - generally the people who would appreciate seating and comfortable waiting areas - don't really have many options.
I appreciate that the open and uncluttered nature of the concourse is probably designed to aid passenger flow and prevent overcrowding, as well as reflect the rather nice flowing architecture, but more seating would do nobody any harm if it was placed strategically - just like the signage in the middle of the floor, which I personally feel could generally be suspended on thin, strong cables from the ceiling.