StringSerializer does not create <string> tag

Consider a method call (API) named 'login' that takes in 4 parameters major version (integer), minor version (integer), user name (string) and password (string). Below code snippet explains how I invoke this remote API.

XmlRpcClient client = new XmlRpcClient();
Object[] loginParams =

{new Integer(2), new Integer(0), new String("user@sample.com"), new String("password")}

;
client.execute("login", loginParams);

The four parameters are wrapped as Object array to method call named 'login'. Below is the XML generated and passed to server hosting login API:

<methodCall>
<methodName>login</methodName>
<params>
<param>
<value>
<i4>2</i4>
</value>
</param>
<param>
<value>
<i4>0</i4>
</value>
</param>
<param>
<value>
user@sample.com
</value>
</param>
<param>
<value>
password
</value>
</param>
</params>
</methodCall>

As you notice the user name - user@sample.com and password - password are serialized without '<string>' tag. Is this intentional. I referred the code StringSerializer and noticed that '<string>' tag is not passed to write method. By XML-RPC specification http://xmlrpc.scripting.com/spec it should have been:
:
<value>
<string>
user@sample.com
</string>
</value>
:

Any specific reason for this change in behavior ?