From Amanda:
I sometimes think that because the filmmakers decided to show us visually Ennis's imaginative projection of what happened to Jack, his version of the story seems more "real" to the viewer or at least more palpable. This doesn't at all make it true... but it makes it powerful.
Agreed to what you and Mikaela said. To see a scene makes it more poweful than only to hear about an incidence (accident in this case) and therefore it seems more real.
Regarding that the visual sense is the most important to humans it's only logical.
I strolled around at Wikipedia a bit and found interesting details about this: The ability of our senses to process information from the outside differs in quantitiy for the different senses. The visual sense is by far the one who is able to process the most information per second. It's around 10 million Bit per second. In comparison, the audio sense processes around 100 000 Bit per second (however this is measured. And why is the measuring unit Bit?).
And I think this is the exact reason
why so many people think at their first viewing that Jack is murdered and don't doubt it. It was my own reaction, too. The moment I saw it, I believed it was the truth. The director's way to show the audience what really happened. Like some omnipotent and omniscient narrator.
But it lasted not long. As soon as I recoverd a bit from being absorbed by the movie, I begun to doubt my impression.
Ang Lee must know this (not necessarily the scientific details, but the fact that the visual sense is the most important). Plus, he said Lureen lied on the phone. Maybe Ang Lee is not totally ambigious about Jack's death? Maybe he let the movie be ambigious about it, but he himself belongs to the "murder-camp"?
Someone once described this question as one of the first symptoms of Brokeback Fever

. Additionally to feeling kind of numb for days it was definately true for me (and for Kerstin, and for my husband, although the latter never evolved Brokeback Fever).