They're being complete and utter idiots.
Unfortunately, I'm not sure we actually have a better option. GreatestJournal apparently has also deleted artists and such in the past, and likes to randomly delete icons - and its servers are robust. And it reportedly also has crap customer service. InsaneJournal has a cap of 100 friends for free users, and has been relatively untested. Journalfen is 18+ and difficult to get an account at, and again, robust servers.
Though, honestly, if I were going with any of them, I'd go with journalfen.
And the thing is, is that for as much as 6A are being ridiculously stupid, most other journal sites are going to have the same issue with underage things - namely because of Congessional and societal pressure. Have you been watching the pressure Congress has been putting on Myspace to ban "child predators"? Yeah. Of course Harry Potter people are going to be caught up in that, because outside of fandom most people simply don't understand the distinction. Oh look, it's Harry Potter having sex - they must be a pedophile. The people don't think "oh, maybe it's Harry Potter as an adult" or even "hey, maybe it's Harry Potter from the last book." I would hazard a guess that most people, when they picture Harry Potter, picture DanRad in Sorcerer's Stone, and don't look any further.
I truly think that moving to another journalling service will be nothing but a stopgap. I have been cross-posting my entries to JF, IJ, and GJ, but frankly I'm not moving from LJ as my main server until I can figure out where everybody else is going. And from what I an tell just by watching my flist, that's going to be awhile.