"Then unsub!"

^{I kid, I kid}

But in all seriousness. The "power user" argument is way exaggerated, and is largely a holdover from the Digg days where an almost cartel-like group of frequent submitters were using their collective influence to push each other's posts higher. This is virtually impossible to do on reddit, which will outright ban users suspected of vote manipulation and making voting rings.

Essentially, the karmawhores here are only frequently spotted because they know what kind of content reddit likes and provide that, not because they shape what reddit likes or thinks by pushing their content. If they post shitty content, they get downvotes like any other user. The only difference between a karmawhore and a regular user is the frequency of submission.

In short, if they wouldn't post it, someone else would. There's countless small-time linkers who will also stumble across images on other sites or on obscure subreddits, and think of sharing something that's popular there but new to reddit. I mean, that is the entire groundwork of this website: people finding stuff to share and sharing it.

That all being said, you really shouldn't be surprised that large subreddits, even this one, display tendencies to upvote and enjoy particular kinds of content. Here, it's a humorous cosplay. It's often also good fanart, official statements by GRRM, etc. And someone is bound to post it at some point. Karmawhores just know where to post the things a certain subreddit likes most.

Tl;dr: karmawhores are an inevitable step up in efficiency for a user-submitted-content-dependent website. Basic supply and demand, in short. But, do not mistake their ability to supply for a supposed capability to alter the demand-side.