It is standard in the Python stdlib that mutation methods usually return None and never echo an input argument.  If one can pass a node to element.removeAttributeNode(node), there is no need to echo it back.  So I suspect that the current behavior is intended.

David, is there a general (perhaps unwritten) rule about how Python translates such functions?

The other 'remove' methods also default to returning None.  Same for the 'set' methods.  All methods would need review before changing just one.