Because if it's not then you can a 24/0/27 build on druid which would be incredibly powerful for healing. I looked but I only saw a topic of debate about it, I found no answer, and when I ask in game no one knows either
Okay so say you have 100 regen and 10 to healing from gear, 10% increase if multiplicative would be 121 regen, 10 increase if additive would be 120 regen. Basically if it's additive, mana cost reduction would be substantially more effective then increase to base of spell talents