I was thinking the same thing at first. i bet even Mageary is thinking damn Joffrey would have just killed them all on the spot for me, but Joffrey was always so short sighted. Like yelling "kill them all!" when someone threw mud at him. That was just a mob of civilians. Tommen was in a much more volatile standoff, and he was smart enough (or maybe just scared enough.. i'm not sure he's all that bright either) to assess the situation, and conclude that they'll find another way.