Well, the issuer has USD / BTC whatever but has to keep full reserves to be willing to cash everyone out in a case of a "run on the issuer".
This is a worst case scenario and realistically they can probably spend half of the USD / BTC / whatever raised on actually running the business.
As the token goes up in price, less and less people will want to buy at the current price. At some point issuer insure themselves against the remote possibility of the price crashing and people making a run on the issuer, by buying out-of-the-money call options if they can find some underwriter.
Then the underwriter's issuing the securities, and the issuer can spend some or all of the money protected by the call options / insurance.
