The reason is because time is already up, but rugby has this good rule which allows play to continue after the 80 mins. That is already a concession. So if you are winning, and time is up, why on earth would you not do that? Why should you gift the opposition a chance to get the ball back in 'dead time'?
Last night in the France New Zealand game New Zealand kicked the ball out and were booed for it, which immediately lost the home supporters any sympathy from me for seeing their team lose narrowly