You can adjust a player's points who is not bought-in to the tournament, and as you noted, it will still show up as $0. The * denotes that the amount was manually adjusted, by the way.
The program COULD display the adjusted points on the Players tab for the non-bought-in player, but it doesn't because players who have not bought-in to a tournament are not included in the Stats. So you'd have a disconnect. Some player who has not bought-in might show -10 points (for not showing up, for example) on the Players tab, but when you ran the Stats, that -10 would not be included in the players statistics.