I don't want my Tristana to focus max HP enemy out of 50 hp enemy and 70 hp enemy. I want her to kill the lowest hp enemies so that only one enemy is left and only one enemy deals damage to my Tristana instead of 3. Even if I lose the round I get less damage to my Mini Legend because there would be only 1 enemy on the board instead of 3.
Imagine this: Let's say you have 2 phantoms (curse enemy unit so that they have 100 HP). Let's say you've cursed tier 3 Vayne with 2 items. She is literally a machine gun. Your whole team focuses that frontline (including your ADCs) while enemy Vayne is in the backline. Quite sucks, Vayne probably will carry the whole team and win the round.
PS. Don't get me wrong, backline is backline and frontline is frontline and I'm not saying that there should be backline targeting option I've just given an example with Vayne. It could be also tier 3 Garen or Darius in the frontline cursed to have 100 HP and then if we set the targeting option to "Lowest HP", our whole team will target him first.
PPS. I don't know if It's a bug or not but sometimes when I have Tristana she attacks random units each AA, doesn't focus one enemy unit.