Except they wanted to be saved because the bells were rung.
Again they rung the bells indicating they wanted to be saved. Dany agreed with Tyrion that if they rung the bells she would stop. Dany had never lied like that before and she has never just torched people without giving them a clear choice. Dany as previously written would have honored her promise to Tyrion and asked the civilians to bend the knee. If and only if they refused would she treat them like the Tarley's.
This was simply an out of character moment. Also Cersei plan was terrible writing. She could simply have placed civilians in the Red Keep and made them actual shields. She did not because of plot induced stupidity. If they were in the Red Keep and Dany burned it down that would still be horrific but would make sense. They didnt do that as this whole season has been ignoring plot and logic for shock value.