[Gameplay] Zz'Rot Portal Exploit?

So it's pretty simple actually. I was playing a Nasus toplane, and the enemy support bought Zz'Rot portal. The enemy support placed the portal in the first bush near the purple side tower after the first two towers toplane had been pushed. That is when I found out that killing a Zz'Rot Portal Minion with Nasus "Siphon Strike" (Q) gives you 3 stacks. What does this mean? Well, per lifetime of a single portal, I was able to get ~50-60 stacks JUST from the Zz'Rot portal minions that came out every 3 seconds or whatever. This happened 3 times, and I ended the game with 960 stacks. Needless to say, my team won. I'm not sure if this is a bug or if it is intended, but I feel as if it should be fixed.

Thanks,

Swuft