It's a safety feature.
In a sudden decceleration, with the seat in the upright position, the seatback will pivot forwards thus reducing the risk of the upper body/face of the passenger behind striking the seatback. To achieve this, the recline mechanism works so that in the upright position the seat is prevented from moving backwards but is free to move forwards.
If the seat is reclined (even slightly) the recline mechanism locks the seatback in place, in a sudden decceleration there is a risk that the passenger behind could injure themselves as they are thrown forward.
View Large View Medium
Photo © Alastair Bor
You can see in the above photo that some seatbacks on the left are folded forward. In this case they have been deliberately pushed forward to provide more space, and its a design feature of the seats - western airline seats don't have that feature, but you see the idea. Obviously the seats wouldn't go that far forward in a sudden decceleration if there was someone sitting in it