thats a rachet system,

-from 1 up to D... then from D to N need to press button, because will be logical you are at high speeds on upshifting procedure, making dangerous to jump accidentally to N

-On normal driving operation, being you beging at D, you can shift from D to N without press button, since rachet system hasn't been engaged

-From parking and Reverse button need to be pressed anytime to any new location.

EDITING... FROM REVERSE TO NEUTRAL YOU DON'T NEED TO PRESS BUTTON, but need to when getting back to reverse... dunno if I'm missing something else