I wonder if it may be that also if you fly pretty much the same flight each time into the USA they don't check (or check very well).
For example, if you live in SYD and go to the USA each time on QF11, would they really care if you don't update the date of departure each time?
In the end, you would need to be denied entry for the **** to really hit the fan for both you and the airline, right? If an airline authorises you to fly on an ESTA which may be incorrectly filled, if the passenger enters the USA fine then there is no problem?
Of course, I'm not advocating people here to cut corners and expect to get away with it all the time. One should attempt to err on the side of caution on these things.