okay, i got the cause of confusion. I believe maverick280857 has considered an axis parallel to the surface. I was visualizing a book lying on a surface - the way we usually place books, and so an axis perpendicular to the surface.
The forces N and Mg act in one plane and the force F in another - both the planes being perpendicular to each other. In such a case, why would their torques add up or cancel? I'm considering say a book kept on a frictionless surface and being pushed by a force parallel to the surface i.e...
But why would the body rotate? The axis is not fixed. Nor is there an opposing force (e.g frictional force in rotation with translation).
In other words, suppose u applied a force into a face of a rectangular block but away from the centre of mass, would the block rotate? The block is free...