Who says the conventional non airfoiled tail surface isn't exerting a lifting force? Kinda depends on what it's angle of attack is doesn't it? As an example I'll use the Mojo 60 that I'm currently enjoying with the CG set so far back that at low airspeeds (5 MPH?), the plane will fly around literally dragging it's tail. Increase the airspeed, or give it a burst of power, and it will level out into normal flight.

Extreme example I know, but I'm able to see what the tail is doing with my own eyeballs everytime I fly the dang thing. Low speed coupled with rear biased CG equals high angle of attack and low lift on the stab. Increase airflow (airspeed) equals increased lift on the stab until it becomes neutral.....

Something like that anyway.

It even seems to me that having an airfoiled (lifting) tail surface would be detrimental. Everytime you'd change the speed, you'd have to retrim the aircraft. Fine in full size aircraft, not so practical in models.