They're not celebrating Bin Laden's death per se, they're celebrating the death of a symbol. While it is in bad taste, and did actually make me a tad uncomfortable, I'd say that in this scenario, it's certainly justified.
To say that this only furthers the cycle of violence is naive, because it's not the celebration that furthers this, but the hunting and murder of terrorist leaders in the first place. The only alternative to that is to sit idly by, which is unfeasible. Murdering murderers for revenge, justice, or whatever you wish to call it, and fostering even more hatred, is certainly not ideal, but there are no real alternatives, in this scenario.