Actually, I believe it IS in the rules. For instance, from Ohio (High School):
BASKETBALL OFFICIALS-COACHES-SCHOOL ADMINISTRATORS
COACHING BOX
In an effort to increase sporting conduct by basketball coaches this year, the OHSAA Board of Control adopted a new bench (Coaching Box) rule which created a 14' box. By now all schools - junior and senior high - should have marked the 14' box with tape or permanent markings.
Team benches shall be located on the scoring table side of the floor. The Coaching box is bounded by the sideline, the team bench, a line at the end of the bench nearest the scorer's table and another line 14' measured back toward the end line. Officials and Head Coaches have been instructed that failure to have a coaching box marked will create a situation where NO coach may stand during the game. School administrators and game site hosts are responsible for complying with this floor marking rule.
COACHING BEHAVIOR
Head Coaches and only Head Coaches are permitted to stand and coach. The Head Coach must remain in the coaching box. When the Head Coach is out of the box and "just coaching" the coaching box rule will be enforced as follows: First Offense - warning and return to box; Next Offense - technical foul.
NCAA - 2012-12
http://ncaambb.arbitersports.com/Groups/104883/Library/files/Basketball Insert_FINAL.pdf
b. Coaching box. Failure to comply with the rule results in a
distinct advantage that is not within the spirit and intent of the
rules. The rule is clear and concise. The head coach or any other
bench personnel may not be outside the prescribed coaching box
except when otherwise permitted by rule. After a warning, for
the first violation, a technical foul should be assessed for any
subsequent infr