Because they believe reducing our energy consumption will save the world regardless if the AGW theory is correct or not.
Take for instance a carbon tax. That will increase the price of transportation of goods, forcing us to buy "local". But this will put a crutch on world trade and globalization. Hansen has actually touted this as a positive social good, although he has not used the word "globolization". If you have noticed the same people who are against globalization are also for AGW.