Not really sure what to think of this in the sense of "why" he's been allowed to return, but it's clearly good news in any case, as he still has widespread support and is the only Haitian (former) President to retain an aura of legitimacy and of working for the people and not foreign powers and oligarchs.
That's surely why George W. Bush got him deposed, the same his father had before.
Aristide returns oddly enough amidst the second round of presidential elections deemed illegitimate and rigged by most Haitians. He also returns after one of the last classical dictators of the American country, Baby Doc, returned some weeks ago, in a move that surprised everybody.
Whatever the case, I have been a supporter of Aristide, fundamentally because he was always the popular choice in Haiti, I even took part in a huge demo in New York in the early 90s for his return. So I still support Aristide's arrival, the same that I could not but reject his removal from power by the US-France imperialist coalition.
Ref. Al Jazeera.