Yes, technically this should have been called a penalty. It is a flagrant violation of the rules. But I suspect if you watch games and look for this it happens quite a bit.
From the Rule Book:
ARTICLE 4. RESTRICTED AREAS
All team personnel must observe the zone restrictions applicable to the bench area and the border rimming the playing field. The only persons permitted within the solid six-foot white border (
1-1) while play is in progress on the field are game officials. For reasons involving the safety of participating players whose actions may carry them out of bounds, officials’ unobstructed coverage of the game, and spectators’ sightlines to the field, the border rules must be observed by all coaches and players in the bench area. Violators are subject to penalty by the officials.
ARTICLE 5. MOVEMENT ON SIDELINES
Coaches and other non-participating team personnel (including uniformed players not in the game at the time) are prohibited from moving laterally along the sidelines any further than the points that are 18 yards from the middle of the bench area (i.e., 32-yard lines to left and right of bench areas when benches are placed on opposite sides of the field). Lateral movement within the bench area must be behind the solid six-foot white border.
Penalty: For illegal acts under Articles 1 through 6 above: Loss of 15 yards from team for whose supposed benefit foul was made. (Unsportsmanlike Conduct.)
