Same reason that people cheered when Hitler died. Or Saddam, or al-zarqawi, or escobar. Because there is one less piece of shit mass murderer breathing our air.
And DON'T give me the predictable "but our president has killed more people than Osama, who are we to blah blah blah blah" Osama bin Laden killed many people, and made even more of them live in fear, acting like a martyr for his cause while he hid in a cave as thousands suffered for his actions.
And I am really not an advocate for the death penalty, but on this one I am almost glad he didn't surrender during his 5 second chance, warranting a bullet right to the dome, way less paperwork this way.
We have wanted him dead or captured for years, now he finally is, and that is a great fucking reason to celebrate in the same streets that he robbed of 2 iconic buildings and almost 3000 people.