This passage is what made the rationale for the decision clear to me: 
"... Until 2009, the Ktunaxa appeared to be on the path to reaching an agreement with the developer, under which compensation would be paid and changes made to reduce the project's environmental impact. But in June of 2009, a Ktunaxa elder said he'd had a revelation..."

I am firmly for the indigenous rights and fairness of the land deals that are being made, but I can't see a sudden "revelation" as a veto-imposing thing, either under the Charter or under the Treaty.