I guess I'm in the minority, but I'm not really buying that this is abusive.
Yes, it would be nice if SO removed the nofollow from known-good posts (which, indeed, it seems they are now doing), but adding nofollow is a pretty simple and reasonable way to make the site a whole lot less attractive to some of the most obnoxious sort of spammers. I would hope nobody is adding links to their SO answers with the expectation of an SEO benefit.
Yes, it would be nice if SO removed the nofollow from known-good posts (which, indeed, it seems they are now doing), but adding nofollow is a pretty simple and reasonable way to make the site a whole lot less attractive to some of the most obnoxious sort of spammers. I would hope nobody is adding links to their SO answers with the expectation of an SEO benefit.