well, for all we know, judgement, common sense, pragmatism, experience and customer consideration, were all used in this instance. Because really, we don't know everything that went on.
And I have witnessed this happen, not hundreds of times, but quite a few. We don't always know where we are in the queue, will the pax have inside info that the cabin crew do not? Even if we were 15th in the queue, as "Iain8867" pointed out, that can, and has changed. Again, not hundreds of times, but a few. The clue that you should be in your seat with your seatbelt fastened, is the fact that the sign above your head is lit. You do not need the seatbelt on just for takeoff, just taxiing round an airport is enough, possibly taxiing faster than it feels like. There has been instances where the pilots have had to slam on the brakes, and just like in your car, if you're not restrained, it may well hurt. We cannot physically stop people from getting up, but once they are up, we have to inform the flightdeck that we are now, not secure.