It also proved how this Superman doesn't care about civilian victims. He has superspeed, supervision, superblabla, and he grabs Zod and crashes him through a gas station that explodes in a giant fireball. Surely the poor gas station clerk, and the people who were just refueling, bit the dust in that split second.
It looked to me like he just got pissed about the attack on his mom and charged Zod, which knocked them back into Smallville. He never took off any higher than a few feet off the ground, and he was traveling through the cornfield most of the time, so I don't think he was aiming for the town or anything. It just happened to be where they wound up when he went off on Zod and attacked.
It's funny how one line in the original film nails it: "General, would you care to step outside?"
You know who else he asked to step outside? The trucker who kicked his ass when he was de-powered. You know how Clark responded once he got his powers back? He went back there to get revenge, spun him around superfast in a chair, and then threw him down the diner counter into a pinball machine where he's knocked out and sitting in shattered glass of the diner that had only just gotten fixed from his last fight, which he could have easily avoided too. Truly, a crowning moment in Superman morality, right there.