While working for 2 previous airlines (one italian, the other british) we had a similar procedure: Seatbelts would be on until reaching about 20,000ft and would be on again when reaching 20,000ft during the descent.
Well in BA we let them out of their seats (in normal situations, when turbulence is not expected) at about 10,000ft and the seatbelt sign goes on again at -10minutes to landing, which also is at about 10,000ft.
I have to say that while in Italy I had less problems with people getting out of their seats when they were not meant to, but in the UK it is something that seems to have affected both airlines despite such a difference in timing the seatbelt sign.
Tightslot might be right in saying that in the US one of the the reason might be because of legal issues....I have also noticed that the sign seem to be on forever!! However my personal experience over here has been very much influenced by how much passengers regard crew in general and how drunk/disruptive/generally unpleasant and uncooperative they were or wanted to be on the day.