DT members like actmyname do tag merit abusers, but only in obvious cases where there's little doubt about what's going on.  I've seen Vod tag people as well. 

In general, though, you are correct that merit abusers don't get tagged.  It's a very gray area and DT action has been less consistent than, say, loan scammers or bounty cheaters and that's because of the inherent uncertainty as to what people consider good posts and how much merit to give out.  Theymos has also suggested that suspected merit abusers shouldn't be tagged, that it'll all work out in the end--and that's why I haven't been leaving negs for people.

Some members have probably ranked up when they shouldn't have, because they're shitposters, but overall it's still MUCH harder for them to do so.  That's progress.