When a RadTreeViewItem is disabled it means that User cannot select it, check it, expand it, collapse it, drag and drop it. This does not mean that you cannot do these operations programmatically. Same is true if these operations are side effect of another operation. Such side effect could be the TriStateLogic - checking a parent enabled node will check all of the child nodes. This does not mean that the disabled child nodes are checked by the User - they are checked by a side effect internal tri-state logic.
What you could do is to implement your own tri-state logic in the ViewModels and check whether an item is disabled before turning its CheckState to Off. You can use this post as a demonstration of making a tri-state behavior in your ViewModels.
the Telerik team
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