Nobody is questioning his legal authority here. That's well established. The issue is, though, that it damages an intelligence channel with a critical ally. It gives Russia info they should not have. It gives them potentially a piece of a puzzle to track down even more specifics. It then exposes that info to other adversaries like Iran.
Then you have the motivations. Was he boasting? Was he acting in the best interest of the US? Did this make the US more or less safe? Did he even think about all of this? At this point, it's just another thing on the pile. There's a reason the WH is in lockdown mode tonight and refusing to clarify anything. They know he F'ed up.