We have rules around a partner navigation property in a recursive containment relationship (it must be single valued and nullable) but we don't call out the fact that the child of a single-valued recursive containment must also be nullable (or the recursion would never end). 

Similarly, we say nothing about the child of a recursive complex-typed property. For example, we don't have a rule against the following definition, although the model it describes results in an endless recursion:

<ComplexType Name="myType">
    <Property Name="nested" Type="myType" Nullable="false">
</ComplexType>