I think it's perfectly reasonable also, however, how do you "see" your target if you are a Cannonneer standing behind a Horde of Troops. I get that you shoot over, I don't get how you "see" to shoot.
True. Everyone who has ever tried to see something level with him through a couple of "ranks" of a crowd will share your opinion. However, those guys in the regiment might just as well not stand shoulder to shoulder. Even in historical formations, be it a legion or a tercio, individuals needed room to use their weapons and shields, their were aisles for arquebusiers to retreat and so on. That wouldn't be nearly enough if the unit was appropriately deep but it is not such a stretch to assume that checking player's LoS is just a testing to see if it is too deep or not. In fact, it is even less conceivable for handgunners who shoot more "linear" (not completely but still) than a cannon to shoot through/over their own...not that it is a good idea at 9 points and -2 to hit to start with.
So, if you are trying to explain "why" TLOS allows artillery to fire through a horde of halberdiers in Warhammer, and you claim that they are actually firing over the infantry....I just don't buy it.
The real reason is probably just the one they put forth: Ease of play. Now admittedly it IS somewhat ironic that many find it such a hassle but there you go.
If my opponent shot his cannon through a unit of his own. i would have no quarels about going right 10 inches from the back of the cannon base then.. and if he goes you cant see that point, i be like well if you go under his right leg, over his hat, around his gun i can see you 
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Well, LoS is a straight line and doesn't go in circles or around edges. Your point stands that it usually goes both ways.
he has built walls in all of his units ( as his theme they are battling in ruins ). so if someone try's to pull the trick they are shooting behind his own troops, he is like well you cant shoot through my unit there is a wall in the way
What trick? The wall and everything else on his base is part of the unit/model. He has to use one point, say, the tip of the barrel from which to draw LoS and not change it over the course of the game, so if he can see from there, you should be able to see this point. Of course it might not matter if he's shooting at a Dragon - the beast cannot shoot back.
Your buddy is modelling for an advantage, which is tantamount to cheating. Tell him that in no uncertain terms. I'm sure someone will post the picture of that Engineer with the telescopic Hochland here in the near future...
