someone reported it in April apparently
(you can see this by trying to report it yourself, it will says there has been a case, and it has been closed)
"Case Log: Mature Tag Needed
- First Report: Mon Apr 20 2015 01:57
Case Closed: Mon Apr 20 2015 21:40
Comment: A member of staff has reviewed this deviation and has determined that it requires a Mature Content tag. A General Mature Content Tag is now in place."
Bare nipples are seen as mature content here on dA, anything showing nipples, genitalia or I think even a bare butt must be marked as mature.
(according to the rules, I'm however not such a prude :') like seriously, it's not even sexual..)