Here is an excellent article stating the principle behind climate action and the Keystone pipeline protests - Stop Making it Worse.
After a year of unprecedented destruction due to weather extremes, the climate fight is no longer just about impacts in the future. It's about physical and moral consequences, now. And Keystone isn't simply a pipeline in the sand for the swelling national climate movement. It's a moral referendum on our willingness to do the simplest thing we must do to avert catastrophic climate disruption: Stop making it worse.
Specifically and categorically, we must cease making large, long-term capital investments in new fossil fuel infrastructure that "locks in" dangerous emission levels for many decades. Keystone is a both a conspicuous example of that kind of investment and a powerful symbol.