I do not think that the tailplane necessarily has to push down for the decalage to exist!
Look at it this way: tailplane stabilizes the plane for example because it stalls after the main wing. When the main wing reaches the stalling AoA, the tailplane is at a smaller, unstalled AoA, so nose falls, tail rises and the main wing is turned to a smaller, unstalled AoA.
In a cruise, the plane might have, say, 6 degrees AoA of main wing - the main wing AoA obviously would not be zero because then the main wing would produce no lift at all and the plane would be in free fall.
Now, for the decalage, the tailplane has to have a smaller AoA than the main wing. When the main wing AoA is +6 degrees, the tailplane might have AoA of -6 degrees, and pushing down, or it might be 0 degrees with no lift, or it might be +3 degrees. Obviously if the main wing is at AoA of +6 degrees and tailplane is at 3 degree angle to tailplane, the tailplane is providing some lift, yet it is still stabilizing the plane. It is only if the tailplane has AoA equal or greater than main wing that the stability vanishes.
How is the angle between main wing and taiplane chosen?