Re: Someone please explain why....
Not to get into an Inception debate here, but whether the totem falls doesn't necessarily even determine with whether the ending is a dream or not. The point of totems wasn't to tell you if you were in a dream or reality, it simply was a guide to help you rule out that you were in someone else's dream. Also, the spinning top (originally Mal's totem) is the only totem that has an abnormal behavior in the dream state. Compare that with Ariadne's totem (the weighted chess pawn), which behaves a certain way in reality that only she's aware of. So it's completely possible that Cobb has merely accepted a dream as his ultimate reality by the film's end (while being in a vegetable state in reality). The top would then fall. Personally I like to think Inception's ending is real, but there's no way for me to definitively prove that.
TDKR's ending on the other hand, is a lot more concrete. A LOT more.