Slapping the backboard is not basket interference. If it is egregious enough, it can be called a technical foul. But it is NOT "goaltending".
"Slapping the backboard is neither basket interference, nor is it goaltending, and points cannot be awarded. A player who strikes a backboard, during a tap, or a try, so forcefully that it cannot be ignored because it is an attempt to draw attention to the player, or a means of venting frustration, may be assessed a technical foul. When a player simply attempts to block a shot, and accidentally slaps the backboard, it is neither a violation, nor is it a technical foul."