This is added in Write.as / WF to prevent instances from having their search rankings ruined by spammers.
A common thing spammers will do is utilize a domain with high search authority to link back to their low-quality domains and boost their rankings. Adding nofollow prevents that, letting Google et al. know that they shouldn’t consider the WF instance linking to their sites as an endorsement.
There would still be some potential for abuse on Write.as since we’re relatively inexpensive, but I’m open to figuring out a way to disable it for Pro users.
As for WF, we’ll have to figure out a good way to do it. Most multi-user admins will probably want the current behavior (many have seen spam problems). But I could imagine disabling it for single-user instances, for example, since the single author is trusted.