Here's PETA's justification:
We’ll often work with a celebrity on an issue that they feel comfortable supporting, whether it’s promoting spaying/neutering or speaking out against products tested on animals. The celebrity may not be an animal rights activist, but their contribution to any of our campaigns is appreciated and we like to commend them for taking a step in the right direction. We have worked with a long list of former fur wearers who eventually had a change of heart about wearing real fur, including Mary Tyler Moore, Anna Nicole Smith, Martha Stewart and Christy Turlington.
Oftentimes, a celebrity gets involved with one issue and then learns more about other issues over time. For example, Alicia Silverstone wasn’t a vegetarian when we first began working with her. Now, she’s a strict vegan and talks about her vegan diet and animal rights in just about every interview. Making the transition to a vegan lifestyle is a gradual process, and we’re happy to work with celebrities on even the most introductory levels. We hope, as you do, that eventually they’ll make the transition.
Please understand that we believe in supporting any effort by any person—celebrity or otherwise—to advocate compassion toward animals. We also believe that by taking a stand against even one form of animal abuse or exploitation—and by helping us to educate the public about these issues—anyone can make a difference for animals.
I can see the point they are trying to make but it doesn't sit well for me personally when an animal advocate group gives publicity to someone who promotes fur.
edit- oops, should have linked the source-
http://www.peta.org/forums/topic.asp?TOPIC_ID=1310