XML Detection

Just for the fun of it I decided to run my XHTML Strict document through
the Validator and override the DOCTYPE to HTML 4.01 Strict. The
validator ran it through as XML anyway. If I take the xmlns declaration
out of my document it runs it through the SGML validator and, as
expected, it fails to validate because of the stylesheet <link /> tag in
the header.

Is this a bug or a feature?

Example:
http://validator.w3.org/check?uri=http%3A%2F%2Fsociety2x03.dhs.org&chars
et=%28detect+automatically%29&doctype=HTML+4.01+Strict