Why?
Government Censorship - far fetched.
Bad publicity and loss of revenues due to protest- far more likely.
Disney/ABC etc have to serve a very wide market of mostly Americans, they have to keep them happy in order to take their money.
A film about 911 that makes such accusations and distortions as Moore has always done in his other works would piss a lot of people off. Maybe not you, but it would piss of a lot of average Disney customer Americans - basically the sort of people not in attendance at Cannes. Because 911 is avery sensitive topic in this country. I'm not sure if its the same in Canada but I assume you are aware that it is so here in the USA...
The difference here is and this is my take on it, is that people on the modern day left are not used to seeing messages they like baing rejected and protested against and so are very upset about this...
We saw this with the huge protests and boycotts of the Dixie Chicks afther they made controversial political comments, the left went apechit over these consumer protests and made all sorts of accusations of free speech violations and censorship. Yet all these people did was exactly what liberals do all the time, protest and boycott buisnesses they disagrered with for ideological or moral or ethical reasosns.