I did not see the specific segment that Elainera saw but, Jeff Toomey, a legal guy, has been commenting on this case since the very beginning for CNN. He has been on Anderson Cooper's second show this week (all week) 10 PM EDT, 7 PM PDT. And I have heard him state the same thing. His explanation makes sense to me but I couldn't possible restate it.
I didn't see the segment, but I think the rationale is that while there may be an ethical obligation to inform on someone you know to be guilty of a crime, it isn't something the state can require. Finding the perpetrator is LE's job. We can't interfere with their work, and need to cooperate if asked, but creating a legal requirement that citizens inform on one another leads us into dicey territory - what if you know your neighbor is smoking pot? Cheating on his taxes? Do we want to live in a state where nearly every friend and loved one is also a potential informant? Because I suspect the majority of people have broken some law, however minor, at some point.
I do think there could be a good argument made for creating a legal obligation to report in certain cases, i.e in cases of capital offenses. There would be precedent as well; coaches and teachers, for instance, are mandated reporters in the case of suspected child abuse, even in the absence of a more sweeping law.
Even if there were such a law, I think it would be hard to convict KTR under it unless there's evidence she actually knew he was the bomber. I think there's certainly reason to suspect she knows more than she's saying, but even if she told him the FBI was after him, she could explain that away by claiming she didn't believe he could possibly be guilty and that she thought LE had the wrong man.