So far, pretty much all political/terror related scenarios end with: "But why has no-one come forward to claim responsibility?"
If there was foul play, why couldn't that have been the *intention*, i.e. making this event be a paradigm shift, just as Sept 11 was?
Prior to Sept 11, hijacks ended with landing & making demands. Which is what the Sept 11 hijackers indeed broadcast to the passengers and to ATC.* But they changed the paradigm, as we know, and those statements were active misinformation.
Why change the paradigm again? To sow the maximum amount of fear and uncertainty into commercial transportation -- make a modern first world jet appear to disappear into thin air, with no ability to find it. (Because whatever happened to it has happened somewhere *well away* from its last known position.) A crash is unsettling, but can be dealt with. A simple disappearance with nothing more -- no debris, no recorded crash, no demands, with everyone looking 1,000s of NM away -- would inflict maximum uncertainty. It already has inflicted a lot, but imagine how much more there would be if the pinging wasn't there, and especially if the pilot had evaded Malaysian radar?
That could be the aim itself. So a lack of any claim of responsibility may no longer be a pointer against foul play.
Remember, bin Laden's strike was symbolically at the three limbs of the USA - economic (4 planes + 2 towers); military (Pentagon); and political (intended target of UA93 was likely the Capitol). This could be a strike at one, the easiest to get to (economic).
* United 93, which had the only CVR to survive, recorded the pax being told "Here's the captain. I would like to tell you all to remain seated. We have a bomb aboard, and we are going back to the airport, and we have our demands. So, please remain quiet." On AA 11, Atta told ATC "We have some planes. Just stay quiet, and you'll be O.K. We are returning to the airport."