As the proprietor of intelligence information, the DCI implements the rules regarding dissemination. Although the DCI could resist turning over the info to the President, such refusal would be frought with career and political problems.
That answer does it for me. I'd assume that Carter figured that if he'd gotten into a hassle with his CIA director over UFO files, there'd be a sensational (and uncontrollable) public demand to learn what was in the files--and a lot of ridicule of Carter for sticking his nose in ("Uh, Jimmy, after your encounter with the attack rabbit, you're now seeing flying saucers?").
