Emperor's Edict on Minister Yin and Yin Sun

Why does the Emperor deliver an edict to Minister of Law regarding Minister Yin and Yin Sun of crimes and plan to exile them to the south, but they show up at the midnight Banquet without being arrested?

He knew about the plot, why wouldn't he know the Empress was plotting against him? Why did he not try to remove them before the Banquet, where they would have had something planned.