Apple has changed its app submission guidelines to specifically ban watch faces from the App Store. It isn't a completely unexpected move given that no hint of this functionality was offered in any of the pre-launch presentations, however its extension to include apps whose primary purpose is to tell the the time has had some developers up in arms.
There are very good reasons for Apple controlling the watch faces that appear on the Watch. Firstly from a litigation point of view they do not need the headache of policing claims and counter claims from real world watch manufacturers that their trade dress rights have been infringed. We've seen this happening in Google's Android Wear store and the results are messy. With Apple's firm stand on protecting design it would be remiss of the company to do anything less.
There's a second reason though. The Watch app store is a new property for Apple and the Watch a new device. Its success is, in part, dependent upon third party software supporting the Watch and supporting it well. Drowning useful software in a glut of watch faces doesn't send out the right message to consumers. Of course seeing somebody with an ill-conceived or badly implemented watch face on their Watch could also damage the reputation of the Watch with consumers.
I don't think this will necessarily be a long-term ban on watch faces. However I believe that Apple has a plan to extract more revenue for the platform by offering additional Watch faces through a separate store or sub-division of the store. With rights holders able to create their own Watch face and sell it at a fixed price. Imagine a band, car maker, sports team or film producer releasing a branded watch face for that product / team / artist, available for download at $1.99 say? The take up would be enormous.
Watch faces could dominate Apple's Watch income the same way that Nokia's XpressOn covers did with dumb phones a decade ago. I believe that's the real reason why Apple is keeping the Watch face its own right now.