People can add your photo to their Foap album without having to buy it.
But why is that?
By adding your photos to their album they increase your chances of selling the photo. If someone buys your photo from their album, they will get $0.25 and you will get $5, so it's a good business for both sides!
You can also add any other user's photos to your albums.
Keep in mind that adding a photo to the album doesn't allow the person who did it to download a photo in the original size as the full size photo is only available for a person who buys your photo!