I was in Singapore for a week of the Haze. I was kind of amazed it was allowed to happen, but the local take (as told to me anyway) was that Indonesia was way too powerful for Singapore to force them to do anything they didn't want to do, and the corruption was such that even fighting the fires effectively wasn't something they particularly wanted to do... and forget about stopping the planters from burning recklessly in the first place.
Not sure if that's accurate but that's what local folks told me.
It's a mix of corruption, poor governance and a powerful palm oil industry in Indonesia. If the USA was next to Indonesia they would have power to influence but Singapore, despite it being a successful country, do not have much power to influence big Indonesia.
Not sure if that's accurate but that's what local folks told me.