Lets talk about Abilities vs. Turrets

I've been wanting to have this conversation with the league of legends community and Riot team for a long time now. I never got to doing it, but after seeing tristanas |e| is able to damage turrets, i decided i would finally create this discussion. What i want to, and want the community to talk about is, as you can tell by the title, abilities vs turrets. Rengars semi overhaul Disabled his Q from damaging turrets. This is very understandable because of the sheer turret taking power he had when doing this, but now i'm a bit confused. With tristana gaining the ability to now take towers very efficiently, along with Nasus retaining his god like tower taking power, I ask why rengar and other champions are having this taken away from them. It can be argued that nasus's Q deals far more damage than rengars Q ever would, but even still could they not change rengars Q to deal less damage to towers? or even remove the gaining of attack speed afterward? This also applies to shen, a champion i feel Riot has put to the side though of course i can be completely wrong. Why would his basic attacks not work toward his passive? The damage from it is far lower than the Q of nasus. This is my question all around, why are champions like rengar, yorick, shen and more not allowed to utilize their abilities/passives against towers or at least tweaked versions of them, when other champions can?