Any game with an 100% discount can't be downloaded in the app because the 'Buy' option is still the only one available. Because this just brings you to a page where you can redirect directly to the download, there is no way to launch the game from the app unless you purchase (i.e. donate to) it.
Any game that is 100% off also doesn't show up in the 'Free' tag on the site; basically 100% discounts haven't been accounted for. :P