I'm with Steve on this one, I have been a DU member for a lot of years and I haven't received the catalog you are talking about, nor any other junk mail I would associate with DU. I don't know about DU but I have noticed that some organizations have a little box to check on the application asking wither they can give your contact info to other organizations. I always check "no".
Beyond that, I believe DU has proven itself to be one of the least contentious, most effective conservation organizations in the history of the world. I do get a lot of other unrelated junk mail and catalogs that I don't want, I just throw them away. I guess that even if DU sold my contact info to other organizations, I would be a little disapointed, but certainly not to the point of quiting them.