I have a module that builds a form that includes a fieldset. Instead of using the <legend> element to render the fieldset title, I want to place this content in a <div> element instead. But I want to change the behavior only for the form returned by my module, so I don't want to place any new functionality into my theme's template.php file.

In mymod.module I have defined:

// custom rendering function for fieldset elements
function theme_mymod_fieldset($element) {
  return 'test';
}

// implement hook_theme
function mymod_theme() {
  return array(
    'mymod_fieldset' => array('arguments' => array('element' => NULL)),
    'mymod_form' => array('arguments' => array())
  );
}

// return a form that is based on the 'Basic Account Info' category of the user profile
function mymod_form() {
  // load the user's profile
  global $user;
  $account = user_load($user->uid);

  // load the profile form, and then edit it
  $form_state = array();
  $form = drupal_retrieve_form('user_profile_form', $form_state, $account, 'Basic Account Info');

  // set the custom #theme function for this fieldset
  $form['Basic Account Info']['#theme'] = 'mymod_fieldset';

  // more form manipulations
  // ...

  return $form;
}

When my page gets rendered, I expected to see the fieldset representing 'Basic Account Info' to be wholly replaced by my test message 'test'. Instead what happens is that the <fieldset> and <legend> elements are rendered as normal, but with the body of the fieldset replaced by the test message instead, like this:

<fieldset>
  <legend>Basic Account Info</legend>
  test
</fieldset>

Why doesn't my #theme function have the chance to replace the entire <fieldset> element? If I wrap a textfield in this function instead, I am able to completely replace the <input> element along with its label. Furthermore, if I provide an override in my site's template.php for theme_fieldset, it works as expected and I am able to completely replace the <fieldset>, so I know it is possible.

What's different about providing #theme functions to fieldsets inside a module?