* It's a well-established social norm (at least in the SF world) that money should always flow from publishers to writers, never the other way around. That's one of the signals that distinguishes reputable journals and publishers from vanity presses. (Google "Yog's Law".)
* As Neil Clarke points out on Twitter, Clarkesworld aims to accept submissions from all over the world. Not everyone can easily make international payments to the US, and not everybody can easily afford what would be a "token fee" in developed countries.
* I'm guessing payment processors wouldn't look too kindly on a business where virtually all of the payments are either refunded to real users, or falsely reported as fraudulent by spammers.