The thrust will either "push" or
"pull", it shouldn't matter which direction the shaft is rotating.

what happens is this. if the driven gear (the prop gear) in the lower unit is
in the foward part of the unit it has a large, easy to use surface to be put to
use as the thurst bearing.

One the other hand, if the driven gear is in the back (i.e. closer to the prop
than the driving gear/drive shaft). This means both gears get in the way of
providing a large, easy-to-use bearing surface.

All of which means unless the design engineers have some compelling reason to
make the tranny drive in either direction they will otherwise just use the
available thurst bearing surfaces as is, using the easiest for forward.