Since it registers itself as a storage provider, it doesn't really need to sync locally. File open/save dialogs in Metro apps have a Dropbox location once the app is installed, same as SkyDrive (or Documents, Photos, etc). That's preferable on RT devices, which this app is probably primarily aimed at.
Most companies don't even have any apps out for their services. There isn't even a Spotify, Rdio, or Pandora apps, and they have a lot more to gain. Releasing Win8 apps is a pretty low priority item for almost everyone.
Well, it really only makes sense to not put a higher priority if the main target user base would be desktop users. However, if they are trying to target Surface RT users or something similar, it would make a lot of sense to go for something that works right now because desktop mode is pretty much unusable without mouse/trackpad.
