Why does Paul Henry argue with people when he is interviewing them?

I'm just watching Paul Henry interview someone from the Warehouse about charging for plastic bags at the check-out. He is also interviewing someone else at the same time who's on the other side of the debate (a climate change sceptic). He interrupts the guy from the Warehouse and seems to argue with him, instead of just asking questions, which is in contrast to how he is interviewing the other guy, with whom he appears to agree. Why does he interview like this? Aren't journalists supposed to be impartial? Or is he not a journalist? Does he only interview people so he can tell the New Zealand people what he thinks about a subject?