5. ... you cannot "cripple" functionality in the plugin and then ask for payment or provide a code to unlock the functionality. All code hosted by WordPress.org servers must be free and fully-functional.
As with all guidelines, rules, and regulations, it's a matter of interpretation.
As I see it: You can only use this plugin with an API key that they provide, that they will only provide to you for free with a wordpress.com login. Rhetorically speaking, what if I'm not hosting my personal blog on wordpress.com?
I hope you see my point.