Clarification on the ng-checked directive - angularjs

Question

I reference the document here: ng-checked
Here, at the example provided at the bottom of the page, you can see that clicking on the first checkbox also checks the second box.
The behaviour that I find with this is that even when the first checkbox is checked, the second checkbox can be unchecked - that is, after checking the first checkbox (the Master checkbox), the second checkbox gets checked automatically. If you now try to uncheck the second checkbox, it works which I find strange. The ng-checked directive is bound to the master model which is still true.
Why then is it possible to uncheck the second box? Shouldn't the ng-checked ensure that the checked state is always in sync with the expression - that is, shouldn't the ng-checked NOT allow the second checkbox to be unchecked as long as the first checkbox is still checked?
How does one manage to keep the second box checked as long as the master checkbox is checked?
Reference Code:
Check me to check both: <input type="checkbox" ng-model="master">
<br/>
<!-- Expecting the following input to be checked as long as the master checkbox
is checked / User should not be able to uncheck this as long as master
is checked -->
<input id="checkSlave" type="checkbox" ng-checked="master">

Answer 1

The ng-checked directive likely (I didn't examine the Angular source code) sets up a $watch on the master property. Only when that property changes will the $watch fire. So when you change the slave checkbox, the $watch doesn't fire, so the change happens.
If you want to keep the second checkbox in sync with the first, add an ng-model to the slave,
<input type="checkbox" ng-model="slave" ng-checked="master">
then force a sync anytime either model changes:
$scope.$watch(
function() { return $scope.master + $scope.slave; }
,function() { $scope.slave = $scope.master; }
);
Fiddle
Note that this will also work:
$scope.$watch('master + slave', function() {
$scope.slave = $scope.master;
});