I always thought an attack action ended your phase because you stopped a moment to aim and it took some degree of concentration. Thats what consumed your phase. Like most of the "attack and move" option have OCV penalties because your effectively "rushing" to move afterward, like Snap Fire and Move By. The character stops a moment to level his hand or ready a proper punch. Making a Half move in combat used to reduce your CV for just the same reason I think, but that rule was dropped. Its very abstract because Hero doesn't have "action points" like some combat systems beyond half and full phase action. Making it "Impossible" without using a Move by/thru or something similar is just a matter of simplicity. Alot of people already bitch that Hero combat is two slow or complex.
In any event if I were institute a Move after you attack rule, I would have it inflict a CV penalty. If I punch someone with the intent of turning completely around and dashing iff, its not going to be as effective as a solid attack, IMO. Defensively, turning my back on the target wouldn't be a good idea either. When you see someone really do that, in my experience, its a quick slap or playfull jab not a all out attack.