I read it and that’s irrelevant to what I said.
You wrote, "but there aren’t any regulations about how close you can put an ammonium nitrate plant?"
And in answer to you question, there are regulations about where you can put an ammonium nitrate plant. The plant owner was violating them. That's how the first sentence answered your question. To wit: "The West, Texas fertilizer plant where a powerful explosion killed at least 14 and injured dozens on Wednesday failed to disclose a massive ammount of ammonium nitrate ordinarily regulated by federal officials, according to Reuters."
They failed to tell the regulators what they had. Whether they did so because they knew it wasn't allowed in the zone they had the business will be determined in the courts, but in answer to your question, it is regulated. I would hazard a guess if the owner wasn't killed in the blast he's going to wish he had been after they're through with him. He might even end up charged with murder or manslaughter for each person killed. I'd bet he's going to be made example (particularly because they want to be able to track those materials for bomb making concerns).