While government regulations might not be the most efficient, they do not cloud a picture of exactly how safe a plant is and will remain that would otherwise be perfectly clear. Short of each investor evaluating safety of each plant, oil rig, and factory themselves, the situation with government regulation removed would boil down to an analogy to the S&P credit rating business. We've seen how for-profit independent third-party rating business has worked out.
If you'd like an example from a market with little regulation, look no further than Bhopal.
I'm not going to argue that the current regulations we have in place are amazing. They're not and they can certainly be improved. I will, however, argue that they do a better job keeping us safer than the free market, as it has been implemented worldwide over the last century, would.