Angularjs ngModel not working with complicate object.

I have set up two demo to replicate this issue.
One is working version with simple object.
Demo http://plnkr.co/Daz7EYzZsxmGX6661Kbc
As you can see, key code is line 36 ng-model="dataItem.firstName". So if u change any firstName in the grid, angular model got updated outside grid.

This is not working Demo http://plnkr.co/xGqJenNDzyKjcmdF1Edm
I have config same code in line 810. But two way binding not working. When i change 1st row in the grid, angular model does not updated but Kendo ObservableArray got updated.

Am i missing something.
I know i can manually update angular scope in onChange event.
But i am curious to know why one work the other one not.
There must be a reason why angular two way binding broken i am keen to know why.
Thx in advance.