CNN, desperate for a startling, controversial headline, made one up, instead of reporting what actually happens. Their headline is true, but it also underscores an us-and-them narrative before all the facts are in:
CNN couldn't just report what Obama did say, which was typically measured. They clearly were ready to go on this whole terrorism narrative, and they stuck to it when the story itself didn't provide the facts they wanted. And that's before we get to the subheadline: "Say we still don't know who's behind Boston bombings." That we bad even if it was good grammar, because it impllies that somehow we do know, even though, as CNN ought to know, no we don't know who was behind these attacks.
Of course, it's very possible that this attack will eventually be classified as "terrorism," and it's possible that it was conducted by a "terrorist group," but we don't know that yet. And even if we did, who cares? Why does Obama have to say the word "terror?"
These breaking news situations are when people often turn to CNN for its fast and fair coverage, but they are losing the fair part of that balance very quickly.