I've just installed that plugin and can confirm that it is doing what you say. Mandatory links that you cannot remove and must pay to get rid of is not allowed. It violates the guidelines for having a plugin hosted here.
See Item 10 here which says
10. The plugin must not embed external links on the public site (like a "powered by" link) without explicitly asking the user's permission. Any such options in the plugin must default to NOT show the link.
Added emphasis is mine. ;)
Can you do me a favor? Send an e-mail to plugins [ at ] wordpress.org and provide them the link to the plugin here and explain that the plugin author is forcing users to pay to remove the link. Put in a link to this topic in that e-mail. That way the plugin will get looked at by the team of volunteers who managed that space.
This is the best way to report plugin issues like that. I'd do it myself but I'm about to run out. ;)