Deleted object would be re-saved by cascade

The question is why we get yhis message? Why NH cant remove item from collection or set property to null in this way?
For example: Objects of type B and C can contain same objects of type A in it list of childs mapped with HasMany().Cascade().All().

class A
{}
class B
{
IList<A> childs { get; set; }
}
class C
{
IList<A> childs { get; set; }
}

Deletion of object B: You will get an error ("...re-saved...") if someone object of type C contain same objects of type A. But if you evict this object C from session cache everything goes fine. I think where is only one reason to generate an error it is then notnull reference present.