I agree, I am also since reading a lot of different viewer's comments here that Cersei wasn't being "nice" for nicety sake; etc. But I will still stick by my own view that Pragmatism, Cunning, and Forethought were never her strong suits EVER. She has always let her emotions rule her judgement and to see this play out is strange not fitting with her character not only in the Books but here on the show as well as you just pointed out.
I will say this Lena Headey is doing a really wonderful job in adding so much complexity to Cersei as she really is much more one-dimensional in the books, I am interested to when we get to the point of her meltdown.