Hehe props to the Erebus player if he did that deliberately to create minions, that was quick thinking by them.
I don't really think it needs changing, you would have still got gold/exp from the reinfocements as if you'd killed them yourself and it's not really so different from him waiting for a reinforcement wave to walk into mines for waiting for his own grunts to die against towers, just more showy.
Yeah, it's a little frustrating that you can't stop him using a side effect of your skill to his partial advantage but it's not like you didnt gain something from him suiciding his reinforcements like that too.
The system should stay consistant, no matter how a grunt dies it should be valid for creating a minion.