It hasn't been frowned upon so far, since I've been far to lazy to do anything about it and it's worked well enough as is. ( Not broken? Don't fix. )
But TFF wasn't created to be a fanfiction archive available to everyone, it's meant to be a prereading tool for writers and a think tank for fanfiction ideas.
The payoff for everyone who isn't a writer, is getting to see shit in the raw, sometimes weeks or months ( Or in my case, even longer then that. *innocent whistle* Lazy bugger, yeahyeah, I know... ) before it's out elsewhere in a more completed form. Another form of payoff, is that some cool idea might be adopted and actually turned from idea into fanfiction by some writer who think that the idea is the best thing since sliced bread.
The tradeoff? Commenting, nitpicking and generally providing the writers with help in finalizing, correcting and finishing their stuff.
Getting the payoff for free? Not so cool in my book... If all that interests somebody is to read stuff, then they can bloody wait until final release on an actual fanfiction archive and not waste bandwidth here.
Some sort of criteria for what constitutes a lurker ( and criteria for what constitutes a fake who posts only to not be labeled a lurker... ), will likely require a fair bit of thought and quite a bit of case-to-case judgement, as well as a version upgrade of the forum software.
Zetaboards has some very nice features, including the ability to quickly and efficiently sort through all the posts a member has made during a specific time period. With that, all that's needed is like five seconds to see if somebody posts something that's actually contributing something to a discussion or if it's just 'Booh-hoo, I do not want to get kickbanned for being a lurker so I am posting pointless stuff'-posts.
However, with so many members, it's still something that'd likely take forever to actually do, especially considering how likely I am to laze about doing something completely different.