Also, why did Daenerys focus on the grain when she burned everything down and not on the Lanister soldiers. They could have used the grain (which was stolen from them to begin with).
Heck, she even complained about losing the grain.
Well I guess there is really no way for her to know that was grain, they sacked Highgarden and they are Lannisters so gold seems to be the obvious. And also it seemed to me she was holding Drogon back, only used to break the lines and burn the carriages but trying to minimize casualties.