Something in the QuickPrep, Im not sure. Instead of trying to activate with a KMS server, it tries to activate with your MAK license.
If you are really curious, you could interrupt a recompose and mount the hidden disk to a different VM to peek at what instructions are in the quickprep sysprep file and compare two VM's with that setting on and off. Its probably a single line in the sysprep unattended xml file.
Another route is to use a custom sysprep file instead of QuickPrep.
