Why did they? That's a good question. As I recall, it was done at the recommendation of the company and other experts. The manufacturer of the chemicals has since said they were not consulted in the decision, and would not have recommended it.
According to NBC, in June, federal investigators have called into question that decision and have begun an investigation: https://www.nbcnews.com/news/us-news/decision-burn-chemicals-ohio-derailment-scrutiny-hearing-rcna90728