The one without Ethernet isn't currently available it is slated for future release.
The one that sold out has 10/100 Ethernet and will be the first that will be massed produced for general use.
I would imagine the one without Ethernet is specifically to be used in Teaching Environment for things like Robotic's and other Control Units where Ethernet isn't necessary or even desirable.
While you may be unable to conceive of any application currently without the need for Ethernet there are plenty of applications in Prototyping Environments and the like as well as Education Environment where Kids will never be in contact with the Net with these devices. After all the Development Platform that you are learning on to write code isn't required to be connected to the Net or for that matter even a Local LAN and I very much doubt that the Lego Robot you just built needs Ethernet Connectivity either.
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