A large part of their difficulties I do believe lie in advertising revenues. With the ads that they host, they'll LOSE that revenue if they continue to allow for M and MA rated stories on the site. By the same token, because of the content WE host, there is no way in hell that their advertisers would ever look at us as a reasonable advertising venue.
Because teenagers are usually rebellious in some way or another, clicking a link stating you're 18 is no real guarantee that the user is 18 or older. Underagers click those links every damn day, regardless of the site.
I think that rather than allowing the M/MA ratings to stay on their site as it causes them issues, they should eliminate them entirely. They have entirely too many children who belong to the site, to where they can safely leave that content there.
They are in the same quandary we are, to a point. In that one of the only ways to guarantee the ages of those who access, is to make it a pay to access site. Of course, neither site can do that (because of hosting fanfiction), nor will this site ever do that.
I agree that they're woefully inadequate at policing their own categories to help prevent what they want to prevent. There are MANY titles that they have categories for that they honestly should not, due to the content of the anime title itself. Anime does not automatically equal a kiddie show, as we all know. If it did, there wouldn't be series such as Gravitation and Mirage of Blaze, now would there?
Parents hold sites responsible all the time, regardless of what their own children do to buck the rules. That is a fact of life, and can't be helped. WE actively search out and delete minors every day. But still, that doesn't prevent them from accessing entirely, nor will it.
As to moderation on that site? They've gotten a LITTLE better over the last year or so, but are still not doing what they need to do. They rely on reports rather than to actively scan what's being added to the site, it looks like. I don't know for sure, but that's what I think, at any rate. Yes, the site is enormous. However, their moderators are paid staff. To me, that means they should be willing to do more than just sit and wait on something being reported to them.