Why does full-text search inside titles return error

When I perform full-text search with Wikipedia API, I can not narrow it to titles only (srwhat=title).

So while search anywhere (default) http://en.wikipedia.org/w/api.php?action=query&list=search&srsearch=sql&srnamespace=14&format=xml

returns results, if I add srwhat=title, I got an error:

<api servedby="mw69">
  <error code="srsearch-title-disabled" info="title search is disabled"/>
</api>

This error is even listed in the docs ( http://www.mediawiki.org/wiki/API:Search ), but without any explanation.