Inconsistent behaviour of parent checkbox when some child nodes are disabled

[Ploppz]

If I have a tree like this and one node (b1) is disabled:

Then if I press the checkbox of p or e, nothing happens.
If there are no disabled nodes, clicking any checkbox will always have some effect.
Thus I argue that the behaviour I see is inconsistent.
I expect that when I click p or e checkbox, it will toggle between selecting all children and unselecting all children (excluding the disabled node of course).