When someone starts a new company, the founder does not 'pre-mine' the shares. The founder issues the shares and then either keeps them, gives some or all away, or sells all or some of them.
With a DAC, the shares are also the internal payment system. So, for the DAC to have any value, users must hold shares in it.
Thus, the founder must give or sell some or all of the shares issued.
Getting the balance right is tricky, which is what the Social Consensus is all about. Give a small but significant proportion of the shares to individuals who have demonstrated a desire to support BitShares by mining PTS or virtually mining AGS, let them make a market in the shares, and then sell some or all of the remainder at the market price.