I agree to an extent, but lets not forget our languages are our tools and not our masters. They are constantly changing and adapting to our trends and our fashions, and have always done so.
I'm from the south of England and live in the North. My children have started saying this which i thought was a little kid thing, but then i recently heard a teacher also say...
"those are the walls of the city they didn't knock down, COULDN'T THEY NOT?" A slight regional difference which at the end of the day might not be grammatically correct, but essentially means the same thing.
I've learnt on my travels around Europe to rally in these colourful little flavours or differences, and not get too wrapped up in what a man made book says.