why does healing wind buff the priest healing anyway?
It's part of the ability...
I feel it's one of the better skills for Sedna, it actually helps a Sed player heal teammates and provide support rather than Inner Grace which only affects the Sed, and Magnificent Presence which provides only a tiny cooldown buff for allies.
Whether it's balanced or not is another question, having a Sedna that is disallowing enemy priest heals through Counter-Healing and buffing friendly ones through Wind II is a great advantage once creep upgrades start rolling out.
I don't see the skill line as being unbalanced or gamebreaking, but maybe people who have to play against it feel differently.