Sorry about any confusion caused. We rushed out two new releases today that completely cleared this up.
You can now install and use Wordfence free. Just install the plugin, go to wordfence.com and get a free API key and you're all set. No credit card required. It's permanently free, no trial period, no cripple-ware or anything silly like that.
The paid API key enables plugin and theme file verification. We also give our paid customers priority support. We'll be adding a few other features in the coming weeks. Mostly stuff that costs us money to provide. The plugin and theme verification uses significant server resources on our end, so that's why we charge for it.
The free version of Wordfence has 90% of the value that Wordfence offers. It includes things like core file verification, malware scanning, checking URLs in your files, posts and comments against google's safe browsing list, checking for malicious code, login security, live traffic including commercial grade geolocation, etc...etc. It's completely awesome and something we're very proud to offer the community.