The problem was that they had thousands of tons of this stuff, all of it rather close to lots of people, all of it designed to kill lots of people, lots of it was unstable, and lots of people wanted to obtain & abuse it. The only apparent way to "make it go away" was to dump it in the sea ("out of sight, out of mind"). They knew it wasn't a good idea, but it was a whole lot better than any other solution they could come up with on short notice. Many decades later we're still postulating "what if" rather than dealing with actual consequences.

Most people similarly discard household toxins: they know throwing batteries/CFLs/paint/etc in the general-purpose dumpster is not "ok", but for them it's an easy & effective solution. Disastrous for someone/something somewhere sometime, but not for "me, now".