Validator complains about onclick in the <area> tag--why?

I've just recently neared the end of a marathon effort to redesign our site to a) use CSS instead of tables b) to validate against HTML 4.01 Transitional. I completed the later task, and then figured it would be worth the effort to move up to XHTML Transitional

It took me about a half hour to bring the page in question into compliance with the exception of two hyperlinks that use some javascript in an onclick attribute, and the validator spit both of these out.

The attribute is valid in HTML 4.01, and I can find no reference to it being removed from the XHTML spec, so why is this failing?

Is it just an oversight in the DTD, or does XHTML not allow this?