Where does the initial steem power for new Steemit users come from?

The white paper specifies how the newly generated steem is allocated amongst holders of steem power (~90%) and content producers/curators/service providers (~10%), but as far as I can tell it doesn't indicate where the steem power that's allocated to new users during signup comes from. It says that new users will start with 10x the minimum required balance to transact weekly. I assume this is also funded via the inflation, but it doesn't seem to be captured by a documented curation award or steem power interest payment. In fact it goes so far as to say "Any initial account balance would have to come from the user creating the account and not from token creation due to the potential for sybil attacks."

Any insight into where this is generated from, and how an attack is prevented in which new user accounts essentially power down their Steem over two years without ever contributing any value?