There are two possible reasons that immediately suggest themselves. The most likely was what those of us who lived through the '60s remember as "guerrilla theater", a form of political protest and performance art in which one proves (for example) that the "pigs" are violent racists by provoking them (e.g. by spitting at them, by throwing urine or feces at them, by threatening them with violence, or by hurling vile insults at them) so that they will, it is hoped, "over-react" in a manner that will confer "victim" status on the initiator(s).
The other less likely possibility is that they are conducting reconnaissance for their compatriots in the jihadi movement (i.e. testing the reactions of the crew and passengers to see whether it's possible to venture back into the hijacking business). Now that I mention it, it's also possible that instead of "reconnaissance" per se, these clowns may also hope that their newfound "victimhood" will result in new legislation or regulations which will prevent crew members or other passengers from interfering in future jihadi operations.
WHY would they do that?
I hate to say, but I'm leaning towards the "testing" hypothesis.
"hey guys. look what we did. and we didn't even get a wrist slap. so if you want to do practice runs, go right ahead, practice setting up your hijack a couple times before you really do it."
Terrorists aren't very creative. I don't think intelligence is really a big requisite for people blowing themselves up.
Does anybody remember the small private aircraft crashing on the white house lawn a couple years before the WTC attack? I remember watching a news story calling for greater air security in "high priority areas", saying things like "if that wasn't an accident, there could have been a lot of damage" and "how secure are our buildings". Not very creative at all.
Terrorists are simple. If it looks like they're trying to soften public opinion to create an opportunity to strike again, they probably are.
I believe the purpose was to generate a lawsuit so the already ridiculous softness and "understanding" that mOslems are treated with are even more soft and unfettered.
The eventuallity is that unless an A-rab is wrapped in explosives and is carrying a detonator, it will be a federal crime to ask him what the purpose of his trip is and why he paid cash for a one-way ticket. Oh wait, that already is a federal crime. Oopsie.