Think about what the true damage means in context of what they are trying to accomplish. Fiora has %max hp true damage so she can duel anyone as long as she plays her cards right, and since she's a top laner where all the tanky fucks are the option was either to give her massive damage boosts to punch through tanks (which makes her actually more damaging against squishy targets than tanks) or the equivalent of 100% armor pen (true damage). The %max hp is there specifically so that she isn't fucked by hp stacking meatheads like volibear and mundo. %max hp true damage makes fiora deal the same amount of damage to tanks as squishies, which means that the playing field is a lot of even when fighting any given champion and it comes down to skill at playing the vitals minigame.
For Syndra, replacing some of her damage with true damage won't really effect her damage to squishy targets (who had low mr anyway, even before void staff/sorc. shoes/abyscal/etc. ), but it lets her punch some actually holes in tanks. Note that it's just the bonus damage which sticks to tanks, and not the whole spell, because she's not supposed to be an anti tank or duelist type champion, but they wanted to buff her a bit and decided to try making her a little bit better vs tanks.
Junglers (and nunu/cho) have smite because they are generally used against non player characters over and over again in game, and it would be unnecessarily annoying to have to make a rough guess all the time, so they decided to make those abilities do less damage then if they weren't true damage bit easier to use.
Vayne's a hunter, her %max hp true damage is like fiora's - it's there so that noone can just walk up to a character who'se entire theme is being a 1v1 fighter and just shrugging off her damage because they bought some tank items.
You should be happy that riven is a warrier who hails from large battlefields, if she was a 1v1 swordfighter she might have gotten true damage at some point too - as it is you get to see what happens when you are forced top build all ad without any true damage and a fed tanky guy catches you (hint, you get rekt because you can't do meaningful damage to them but they can squish you).