Anomaly in visibility of id in RSS?

[davmo]

When parsing RSS the id method sometimes returns the link.
This seems to depend on whether isPermaLink is true or false. See the demonstration below.

Is this expected behaviour? If so, why?

Modified t/samples/rss20.xml:

<?xml version="1.0" encoding="utf-8"?>
<rss version="2.0" xmlns:xhtml="http://www.w3.org/1999/xhtml">
<channel>
<title>First Weblog</title>
<link>http://localhost/weblog/</link>
<description>This is a test weblog.</description>
<language>en-us</language>
<copyright>Copyright 2004</copyright>
<lastBuildDate>Sat, 29 May 2004 23:39:25 -0800</lastBuildDate>
<pubDate>Sat, 29 May 2004 23:39:57 -0800</pubDate>
<generator>http://www.movabletype.org/?v=3.0D</generator>
<docs>http://blogs.law.harvard.edu/tech/rss</docs> 
<webMaster>Melody</webMaster>

<item>
<title>Entry Two - link and id differ, isPermaLink="true"</title>
<description>Hello!...</description>
<xhtml:body><![CDATA[<p>Hello!</p>]]></xhtml:body>
<link>http://localhost/weblog/2004/05/entry_two.html</link>
<author>Melody</author>
<guid isPermaLink="true">http://localhost/weblog/2004/05/alternative_url.html</guid>
<category>Travel</category>
<pubDate>Sat, 29 May 2004 23:39:25 -0800</pubDate>
</item>

<item>
<title>Entry Two - link and id differ, isPermaLink="false"</title>
<description>Hello!...</description>
<xhtml:body><![CDATA[<p>Hello!</p>]]></xhtml:body>
<link>http://localhost/weblog/2004/05/entry_two.html</link>
<author>Melody</author>
<guid isPermaLink="false">http://localhost/weblog/2004/05/alternative_url.html</guid>
<category>Travel</category>
<pubDate>Sat, 29 May 2004 23:39:25 -0800</pubDate>
</item>

</channel>
</rss>

Processed with eg/check_feed.pl:

Title:     First Weblog
Tagline:   This is a test weblog.
Format:    RSS 2.0
Author:    Melody
Link:      http://localhost/weblog/
Base:      
Language:  en-us
Copyright: Copyright 2004
Modified:  2004-05-29T23:39:57
Generator: http://www.movabletype.org/?v=3.0D

    Link:      http://localhost/weblog/2004/05/entry_two.html
    Author:    Melody
    Title:     Entry Two - link and id differ, isPermaLink="true"
    Caregory:  Travel
    Id:        http://localhost/weblog/2004/05/entry_two.html
    Issued:    2004-05-29T23:39:25
    Modified:  
    Lat:       
    Long:      
    Format:    RSS 2.0
    Tags:      Travel
    Enclosure: 
    Summary:   Hello!...
    Content:   <p>Hello!</p>

    Link:      http://localhost/weblog/2004/05/entry_two.html
    Author:    Melody
    Title:     Entry Two - link and id differ, isPermaLink="false"
    Caregory:  Travel
    Id:        http://localhost/weblog/2004/05/alternative_url.html
    Issued:    2004-05-29T23:39:25
    Modified:  
    Lat:       
    Long:      
    Format:    RSS 2.0
    Tags:      Travel
    Enclosure: 
    Summary:   Hello!...
    Content:   <p>Hello!</p>