Bin Laden is the stuff myths are made off, either the bogeyman myth of the West (which is largely played out now) or the hero legend of some people in the Middle East.
Every legend has to have an end, the monster slain, the hero rests to return at the appointed hour.
By giving Bin Laden an off screen, unverifiable death by assassination the hero legend (the most dangerous of the two for us) has it's finishing flourish.
If he was put on trial, evidence was brought forward and he was exposed to the world as a human being and not killed, the hero legend would have been largely dissolved and the bogeyman myth with have been exorcised with the Americans seen as the good guys.
Justice was not served by assassinating him, revenge 'maybe'. We pride ourselves that people are considered innocent until proven guilty in which case he technically died innocent regardless of whatever he actually did in life.
Since the death of Kennedy the majority of people have a degree of skepticism about what our governments are up to (the Prism scandal just confirms what we often suspect).
Having the Bogeyman killed out of view, without trial, posting fake photos and then retracting them and burying the body at sea is playing into that speculation. If we can be skeptical you can bet his followers are beyond skeptical.