It looks like REI no longer provides financial support to the Sierra Club.
http://activistcash.com/index_foundations.cfm?alpha=R
http://www.allcrawl.com/read.php?2,472
Many organizations have severed their ties to the Sierra Club -- granted the organization still has many supporters. Around 2003, a character named Paul Watson attempted a "take over" of leadership and policy for the Sierra Club. Paul is an extremist and has a history of violent activism.
Paul's own words... "There's nothing wrong with being a terrorist, as long as you win. Then you write history."
From what I read, Paul is a terrorist and should be shot down like Bin Laden. Paul's run as a high level official in the Sierra Club only lasted several years, but the organization has been extra extreme ever since his tenure.
After looking into the activities of the Sierra Club, I can't imagine a reasonable person supporting. However, it is difficult not patronizing organizations to provide them any type of support. It appears that many large companies send some type of pay-off to the Sierra Club.
As for REI, I still won’t shop there for just for spite.