Now, I don't know if this is new or not, but their FAQ page says that the reason they're now free is that some of the wallpapers that will show up on your desktop (the app periodically changes your wallpaper for you, as often as you'd like) are "beautiful desktops that reflect a brand." I haven't yet seen any obvious ones pop up in the old version, and I haven't started using the new version yet, but I guess there's a chance that you could have, say, a Wal-mart desktop show up. That would probably give me reason to uninstall the software right there, but of course that's up to you -- if you don't like it, you could always try Desktopia, or just do it yourself with Automator.
I haven't had that problem yet, though -- in my experience, Desktoptopia has provided some great wallpapers (with no brands I could notice) as regularly as I'd like with no fuss at all. If you want some change to come regularly to your desktop wallpaper, but don't want to go hunting around for the right pics, Desktoptopia is a great solution that is now completely free. Great to hear.