In "Looper" was there a reason why they didn't send people directly to the furnace?

I think the intention is that it's a lot easier and cleaner to have someone on the other side there to make sure the victim is taken care of, rather than just popping them to a location that they'll die at.

Think about it this way. Lets say that they did pop people into a furnace to kill them. What happens if said furnace is down that day and the victim doesn't get killed? Now they've got a royal mess on their hands.

Ok, so a man-made way to kill them doesn't work, lets pop them over some lava. Can they do that? Transport someone mid-air? That doesn't seem likely as we always see them touching the ground when they appear, but we can't be sure of the laws. OK, the bottom of the ocean? Well that's submersing them in a non-air medium, which might not be possible.

So, out of all of the possibilites I can think of, transporting someone back to a known spot in time to be killed by someone seems the most logical, especially since you have someone back there governing everything.

For the same reason Batman lived as long as he did; when no one is there to supervise the death, there's no way to be sure the death actually occurred. Yes, I guess they could have someone sitting outside the blast furnace door peeking through a window to make sure the transport took place, but why bother? Sticking them in a field and knowing they're gonna get popped with a shotgun is probably just as easy and much more easily confirmed.