"Pollute their water"
Pipelines are run literally hundreds of feet below the water bed to prevent this from happening. What a waste of money and resources for absolutely no reason whatsoever.
Not to mention that it didn't even go through their land in the first place
The fear is that the construction/pipes could leak oil and other things into the river, which runs through their territory. The pipeline would not enter, but it could pollute water that does.
I'm not actually entirely sure on whether this was a reasonable fight, but I feel obligated to support it after the stuff these guys have had to go through. Throwing tear-gas, firing rounds at, spraying freezing water in subzero temperature and beating/arresting peaceful protesters (many of which literally just sit down and pray) is insane and instantly isolates people from a cause. I actually know a girl who was arrested with a group for helping to form a hand-circle around some praying Native Americans.
Misguided people with their hearts in the right place are MUCH easier to support than corporate starfishs willing to do what they want by any means possible. Also, I love to see a victory for protesters against massive corporations because it sets a powerful grassroots precedent. Like it or not, this can be cited as an example that direct action from real people can change things from now on.