I suspect that passengers stand up before they are supposed to because:
a) They know that the nanosecond the seatbelt sign goes of everyone will stand up at the same time, desperate to grab their stuff, they're just trying to beat the crowd.
b) They don't appreciate that the a/c may move unpredictably once it's on terra firma or what speed they are still travelling at.
c) They don't believe anything nasty can happen to them when they're not in the air. Let's face it, once they're off the plane they're often packed into a bus on which they're expected to stand up while the Michael Schumacher wannabe at the wheel attempts to break the land speed record, and the plane feels like a much safer environment than that.
Frustrating though, I'll bet.
B&t