Why is self-healing reduced on dominion, but not shields?
With the ridiculous cooldowns you can get, it makes champions with shields way too overpowered. Look at Diana for example. She can pretty much outsustain any damage sent her way, even vs melee dps late game.

EDIT (for clarity, a few additional points I made in further posts in this thread):
- I am all for keeping the healing reduction on life steal and spell vamp, I am criticizing the fact that champions with specific self-heal abilities are unfairly nerfed (especially compared to already overpowered shields). Since, unlike life steal/spell vamp (which are primarily gained through items), abilities are obviously set in stone for a champion.
- Nome seems to have (partially) realized this, which is why he removed the healing reduction on ally heals. Why not go the logical step further and remove it from all non-lifesteal/spellvamp abilities?
- An additional point: Health Relics by themselves devalue healing abilities because they make healing available to all champions regardless of their skill set.

Because healing is a permanent increase to your life, while shielding is temporary and thus has much more counterplay. Baiting the shield or waiting until the duration runs out to engage, for example.

Sorry, no. With max cdr (boots, bft and masteries) Diana can have her shield up permanently. Champions with self-heal abilities rely on that heal as a "shield".
Also, i am not talking about spell vamp or life steal. I think it's totally fine they have a reduced effect on dominion. My problem is that certain champs with built in self-heal are just unfairly nerfed in dominion.

And what's more, Nome seems to have recognized this by removing the reduction on healing abilities that affect other champs. Why not go the logical step and remove the reduction on all heals (be they self or allied) as long as they aren't life steal or spell vamp?

You can still heal others for the full amount...

I guess you really want Soraka bot?

it's there to keep some self heals such as Mundo's ult or Volibears passive from being too strong.

PS: I thought it was obvious but a shield isn't going to take you from 50 HP to 2500 HP. Heals will get you there.

Have you ever played vs Nunu bot? That is why the reduction is there. Also, to keep lifesteal in check.