TD scaling with the enemy in general would just piss people off. True damage scales only rarely at all, and when it does it's NEVER with an enemy stat.
Nunu: Consume does true damage but can only be cast on minions.
Irelia: Flat amount on autoattacks from W, no scaling with stats
Olaf: Flat amount as an ability. Does true damage back to him.
Twitch: Per-stack DoT, applied on autoattacks. Scales slightly with level. Coincidentally, everyone hates the rat.
Corki: 10 percent of his own damage on autoattacks
Cho'gath: Recently-nerfed ultimate. Scales at .7 of AP but nobody builds enough AP to reach the levels he could previously do anyway. Was a melee-range, extremely powerful nuke. Known and loved/hated for being the only thing of its type.
Kog'maw: His passive makes him a true damage bomb on death. Scales with his level.
If you could punish MR with True Damage, I could see teams of a tank or two, 2-3 AP Nukers, and 1 Kog or Corki building around that item to put teams in impossible dilemmas being a common thing in arranged teams.
Things that scale with the victim (Jarvan's passive, Mundo's cleavers, Veigar's ultimate) are always very powerful and very hard to balance because they punish the enemy severely for building particular things.