Major Description
From the spec document page 5-12
"For <map-entries>:
1. Call the property getter, if it exists.
2. If the getter returns null or doesn't exist, create a java.util.HashMap, otherwise use the returned java.util.Map.
3. Add all entries defined by nested <map-entry> elements in the order they are listed, converting key values defined by nested <key> elements to the type defined by <key-class> and entry values defined by nested <value> elements to the type defined by <value-class>. If a value is given as a value binding expression, evaluate the reference and store the result, converting to <valueclass> if necessary. If <key-class> and/or <value-class> are not defined, use java.lang.String. Add null for each <null-value> element.
4. If a new java.util.Map was created in step 2), set the property by calling the setter method, or log an error if there is no setter method."
MyFaces is skipping step 1 and creates a new HashMap instance in any case.