Explain that one to me again?
Because shit happens. I've worked at several big chemical plants and all of them have had spills. (To me, this sounds like a "spill" and not "dumping waste.") It's just the nature of the beast, nothing works perfectly all the time. At one plant in particular, vandals/kids/idiots with too much time on their hands got onto the property (not hard to do when the facility covers thousands of acres) and removed a cover off a pipe, causing thousands of gallons of water with a ph of about 1 to flow into a nearby stream, which eventually made its way into the bay and caused a large fish kill. Yes, the company was fined. Yes, corrective action was taken to avoid it from happening again.
From what I read, Exxon cleaned up the contaminated area as best they could. I seriously doubt the spill was done on purpose. I live in the middle of frack-land and these oil companies are spending millions buying/leasing mineral rights, hauling equipment in and out, drilling, fracking, trucking out wastewater and hauling equipment away. Millions of dollars are spent at each drill site. They're not going to risk "dumping" wastewater to save a few bucks on having it hauled away.