Why does soraka's Q still heal her if you have Banshee's up?

actually that is consistent with secondary self buff effects of spells

eg. 's attack speed is increased when he breaks your banshee

heals for his q's on your banshee

It really should, Raka being shut down by Banshee's would not be fun.

Banshee's is there to protect you from harmful stuff not negate benefits for opponents.

Rakas passive is if it hits an enemy champ which it does it doesn't have to hurt you. Banshees just absorbs the effect on you not prevents the ability from being cast on you.