Is that the case even if they were already shaken when I entered the hex? If I enter into a melee with a shaken unit it is automatically eliminated so why wouldn't a tank do the same thing?
Basically I'm driving down the road. A shaken unit is in the way so I run over them and continue on my way to conduct an AM against the unit I am after. Seeing as the shaken unit can't fight back surely they just get crushed by my tank. I can't possibly have to drive around them do I?
In a Melee, you have to stop in the hex, thus your troops are in the process of stabbing/shooting the Shaken enemies to death, etc. You don't get to roll right through (unless you have Running Killer Skill).
In the Overrun, you still have to spend the extra 4 MPs (read: effort to kill those enemy troops) to enter and to move through the hex. Going around them may cost less than the extra 4 MPs to do the Overrun, maybe not. You have to make a tactical decision.
Plus, rules are rules. You can't AM into an Overrun b/c you are firing your weapons at the unit you would be overrunning and not saving your ammo for the target after you finish your movement.