They don't like fusion because like jasper says "its just a cheat tatic to make weak gems stronger" they don't like fusion because it makes gems stronger then what they are. They want gems to have naturally strength.

Exspecially if both gems are strong. Like me, I'm alittle too strong on my own. I need somebody to help crontroll my anger and rage and stop me from destroying things. That's were sapphire comes in.