This whole thing is merely a wait and see political move (not that I think JS will ever bend the knee; especially considering the confirmation of his heritage). He knows what he must do to beat the Walkers. He's not being defiant in the traditional sense. I think you are misunderstanding Dany's motivations and moves. Comparing this situation to the High Sparrow issue is like comparing a Dothraki horse to a barrel of Dornish wine. Besides, I think you are also forgetting how Cersei dealt with the High Sparrow. She roasted him, some of her enemies, and hundreds of innocent people. Plus whoever was nearby when the building exploded. Kinda like comparing a lit match to a raging bonfire, is it not?
This is a medieval setting. With resurrected beings. And wargs. And beheadings. And rape. And poisoning. And dragons who like to char their meals before eating. And And beheadings. And frozen yet fast-running bloodthirsty zombies. And the ever popular testicle removal. And a Lannister family who is responsible for much of the continent's current misery. Joffery beheaded Ned. The Lannisters even made it look like Ned was a villain to all of Westeros so that crowds would cheer when his skull hit the platform below. Oh, and how can I forget the Red Wedding? You have a problem *plotwise and/or strategywise* with Dany BBQing a couple of asses that refused to serve her? Did you have a concern when she had the slave owners killed back on Essos?
The Tarley BBQ is a move to show Cersei that Dany means business. Dany could have roasted ALL of the defeated soldiers. Sure, she can use them better living than dead.
Dany knows she has to bust a few heads BEYOND the normal spoils of war in order to meet her goals. She refuses to be as ruthless as, say, Cersei. She refuses to be like her father. She refuses to be like her brother (the one that Khal Drogo killed). I guess killing the 2 Tarleys automatically negates everything she did for the thousands of slaves she freed *sigh*.