Substition?

[Clark Kent]

3-3-1
A substitute who desires to enter shall report to the scorer, giving his/her number.
a. Between quarters, at halftime and during a time-out, the substitute must report or be in position to report to the scorer, prior to the warning signal which is sounded 15 seconds before the end of the intermission or the time-out

So my real question is why? Why can A6 enter the game when there are free throws to be administered but if it were an out of bounds play A6 wouldn't be allowed in the game if he/she failed to report to the table prior to the warning horn?

And what rule (or lack of rule) changes the status of the substitute to become a player?