Microsoft screwed up and made it free on their store. Act fast before someone realizes and changes it. I don't even own an XBone but I got it anyway in case I do later. It's only been like this for 4 hours at most so act now.
Edit: This may be a free weekend kind of deal but there is nothing official to confirm that and it doesn't say so on the store page. Also free weekends on Xbox are usually for Gold Xbox Live members only. Oh well, I guess we'll find out.