What determines the direction in which motor proteins go?
While you're entirely correct that the ABABAB sequence is symmetric to a motor protein cruising along, the individual $\alpha$ and $\beta$ monomers themselves are not symmetric along the axis of movement (or any axis at all, as far as I can tell). This produces a polarity that can be seen in the figure below, from the original 1998 paper describing it.



The figure you use above to display the microtubule is slightly misleading in this way; it shows the subunits as blobby spheres. However, the motor proteins are able to distinguish the top of these molecules from the bottom, seeking a unique set of loops and helices to attach to. As it walks, then, the motor protein can only bind to one side (pole) of the subunit and is unable to walk backwards.