Jetpack Related Posts use nofollow for 2 main reasons:
On WordPress.com, we track clicks on the Related Posts so that we can improve the algorithm used to find related posts. By using nofollow, we prevent crawlers from affecting the algorithm.
We also use nofollow to dissociate the related posts from the current page. Since Related Posts are automatically generated, the nofollow should prevent the module links from affecting SEO on either page. We try to make sure that the module doesn’t hurt your SEO, just in case unrelated posts would appear in the module.