(Spoilers Extended) Option that Dany and Co. Haven't Brought Up.

I am sitting her doing my obligatory post episode browsing/reading this morning and a thought came to me: It has been hammered over and over that Dany doesn't want to be 'Queen of Ashes' by taking her dragons and burning every castle/keep to the ground. Can someone explain to me why she can simply take her dragons, fly to Kings landing and just burn the hell out of ONLY the Red Keep? She doesn't have to kill everyone in the city to take out Cersei and her loyalists?

Maybe it is the same reasoning as not using the dragons at all, but it seems to me, you could give the decree that if they don't bend the knee you are going to burn the Red Keep to the ground (ala Harenhall & Haren the Black). We've already had the 'Field of Fire II' - Why not mirror Aegon taking Harenhall?