Because they, really, know that DDT is, both, harmless and beneficial?
Maybe in light quanitities, but countless barrels of it in a concentrated area is surely not. As they noted and as noted over the years, DDT is showing up in the fish and animals, cancers are up, etc. Sorry, can’t sweep that under the rug or, here, bury it at sea. They need to clean it up and explain why that haven’t already done so.