Can someone explain to me why this does not work? I understand why a Titan coming into play would not work with a Torpor Orb out, but if Riku is already in play short of stopping the creature from coming into play I do not seem to be able to wrap my brain around this. Thanks!
"Creatures entering the battlefield don't cause abilities to trigger."
This applies to any triggered ability that would trigger due to a creature entering the battlefield, be it on the creature itself or on another source.
From the Oracle rulings:
6/1/2011 Torpor Orb stops a creature's own enters-the-battlefield triggered abilities as well as other triggered abilities that would trigger when a creature enters the battlefield.
6/1/2011 The trigger event doesn't have to specify "creatures" entering the battlefield. For example, Amulet of Vigor says "Whenever a permanent enters the battlefield tapped and under your control, untap it." If a creature enters the battlefield tapped and under your control, Amulet of Vigor would not trigger. If a land (that isn't also a creature) enters the battlefield tapped and under your control, Amulet of Vigor would trigger.
Notice the second ruling means Dryad Arbor
entering won't trigger Rampaging Baloth