Mantis Spam Accounts?

We've been running a copy of Mantis here in the Teaching & Learning Centre to track bugs and issues in our projects for a couple of years now. And over the last few months, there have been a couple of accounts created per day in an attempt to proliferate spam. They create an account, with the URL pointing somewhere spamworthy, and then never post any content.

Does anyone know why someone would try to target spam at Mantis? I can't see how that would gain them Google juice, so I'm boggled that someone's taken the time to tweak a bot to hit Mantis. 

I'll occasionally go in and prune out these orphaned accounts, and make sure they haven't lefk any content, but so far it's been a strange but harmless(?) exercise.