This was "codename" classified information, the highest level. When FDR died and Truman was sworn in, he was informed about the Manhattan experiment-- the vice-president was not looped in. That's how secret the information we're talking about is.
Moreover, it's not about sharing info to stop terrorists, it's about burning an intelligence asset inside ISIS, and possibly getting him or her killed. And not even a US asset; one from one of our allies, reportedly Israel. This will make our allies less likely to share intel in the future and make us all less safe.