They're technically using it for it's purpose, or they can claim they are even if they have a different motive. Stackoverflow allows links in comments and posts and this could be abused.
Sites with high pagerank that allow user submitted links are few and far between. If they didn't put nofollow (at least for new accounts), SO is popular enough that it would have to deal with targeted spam attacks aimed specifically at them. That is a sucky problem to deal with.