This Pickpocket mastery sounds very similar to my concept from the "Synergy" megathread which refused to die. In my version, champions gained gold for affecting another champion (ally or enemy) with a spell. This made more sense to me, as it allowed for defensive supports like Nunu to still get more gold.
One issue I have with your version of Pickpocket is that, while it seems to reward aggressive play in a way which I like, it also will tend to have a snowballing effect: if you're getting zoned or harassed under your tower, the enemy bot lane will already be ahead in the lane AND they'll be getting more gold from this mastery than you are.
So what is the reasoning behind using autos, which clearly disadvantages certain supports?