LOS on slopes

[Blackcloud6]

. Disregarding the reality of hills (I see hills all the time in Colorado.),

This is an interesting aspect of the problem of trying to make 3D real terrain work in 2D map and keep it simple.  A friend and I once took the Last Chance for Victory game maps to Gettysburg to compare the maps to the actual terrain.  LCV has three levels of rules to use for LoS in the game, the latter one being very complex.  We were struck by how accurate the map and how in the gently rolling terrain how much you could see from standing n the reverse of a crest.   So, once again, the answer becomes "it depends."  It depends on the type of hills, their slope, their height, the vegetation and so on.    It comes down to what the designer decided to do.
 

have you got a justification within the rules?

This!   The rules rule.  At the end of the day, we have to play by the rules of the game.  The designer of the PG LoS rules made a decision that units can fire from the reverse side of what we perceive is to be the crest.   And it really does not matter why he did so.  It simply could have been done for simplicity's sake or it may also be a "design for effect.  And, as I suspect, it is a combination of the two and there maybe many other reasons.   

I admit, the rule is not fully intuitive because our eyes and brain see the units on what we perceive to be on the back side of the slope and thus we make the very same error I made earlier in the thread.  But, as I recall, I usually reread the LoS spotting rules and examples whenever I play in such terrain.  It takes a few seconds, y the brain cells are refreshed, and one can then go on and play and play accurately.

I say this to many a new ASL players: don't fight the rulebook, fight the enemy!  The same can be said for PG.  Your gaming experience will be better, IMHO.  And PG is not nearly as complex as ASL.