Q:

Why is the net torque 0 when pushing a block across a table?

The way I see it, when I push a block across a table, taking care to push in line with its centre of mass or just below, the object should topple over because the friction should create a (clockwise, in the case of the diagram) torque about the object that is not balanced out by anything else.
However, there must clearly be another force acting on the block above or below its centre of mass with exactly enough of a magnitude to balance out the torque caused by friction, hence resulting in a net torque of 0.
What is this mystery force? 

A:

The confusion arises because you are thinking of one force acting from the floor. In reality there is a pressure distribution on the contact that is often approximated by two forces A and B

They arrange themselves such that the net torque is zero because the block is not rotationally accelerating. If you push a block over an edge and let it fall the net torque will not be zero and the block will tilt.

