The best explanation I've heard is that at high angles of attack the rudder is nearly vertical providing the pilot better yaw authority. I don't know whether this is true or not but it makes for a good story, doesn't it?
The story goes something like this:
As the plane pitches up, the rudder, being slanted "forward," moves closer to a vertical position and provides better directional control at low speeds. Conversely, at high angles of attack a swept tail (and rudder) moves further away from a vertical position and is, in theory, less effective.
Some go on to speculate that the leading edge of the vertical stab, being un-swept, protrudes further into undisturbed airflow at high AOA/low airspeeds which may slightly enhance the pilot's ability to control yaw at the edge of a stall.
I've never spun a Mooney (I did complete my CFI in an M20F/201) nor do I have a burning desire to do so. I imagine that it's not a friendly spinner. Catching the spin early would be critical in a slippery air frame like a Mooney.
A brief online search of Mooney tail lore yields everything from pure marketing to discussion of wetted area to better directional control during landing to spin recovery/prevention to "those swept tails provide no speed benefit and are simply designed to look good."
Here is one discussion (of many) on a Mooney forum regarding the tail design: http://mooneyspace.com/topic/9905-mooney-tail-aerodynamics-not-backwards/
I doubt very much that it's simply a marketing gimmick (although, it could be as simple as that!).
I believe that the explanation I've relayed here is merely "somewhat plausible" as it was gleaned from hours of beer-fueled hangar talk and some Google searching...not exactly a scientifically rigorous method of research. I have no credible references and, as far as I know, no data exists to back any of it up.