I don't want to sound high and mighty, but I think it's ending was for people who like film form, instead of just liking film as entertainment. The way that the narrative shifts completely from the death of all the people to a jolly country song and hilarious joke created by the situation from the beginning ("Who's the gun for?", "Well, the niggers!") is a way of exploiting the audience by dragging them away from the horror genre completely, and putting them in an awkward situation. When I saw it in the cinema, people weren't sure whether to laugh or not (I was laughing) because they didn't like the fact they'd be laughing at jokes and the situation when ALL of the protagonists had died.
Also, when the main character gets shot to pieces at the end, it's meant to be taken in two ways. It could be seen as "that's what was coming to him", or it could taken in the comedy sense that we have just put so much time and interest in this character, just for the director to kill him off in an utterly unrelated way.
The original "Night of the Living Dead" uses this in a similar, less funny way. For those who haven't seen it, the main character manages to survive the whole night of zombies, just to be shot by a group of people out to kill all the zombies. We had never seen these people before, and all they did was come up and fire one shot into the head of this person, and moved on. The director of Cabin Fever could be paying homage to this film, but he also makes it more comic in the process (which I liked).