Each technical foul charged to any bench personnel other than the head coach is also indirectly charged to the head coach. This includes technicals on assistant coaches, trainers, team members, statisticians, towel boys, etc. During pre-game warmups and at halftime all players are bench personnel for purposes of this rule. After three indirect technicals, the head coach is ejected.
The notion of indirect technical fouls makes the behavior of bench personnel the head coach's problem and provides an incentive to keep them in line. However, as Snaqs pointed out, if indirect technicals counted toward the team foul count it would penalize a team twice for a single act.