TYPO3 Core

Problem still exists in TYPO3 CMS 6.2.25 but not in TYPO3 CMS 7.6.9.

6.2 contained before rewrite of form engine in 7.6 in class FormEngine:

// Set field to read-only if configured for translated records to show default language content as readonly
if ($PA['fieldConf']['l10n_display'] && GeneralUtility::inList($PA['fieldConf']['l10n_display'], 'defaultAsReadonly') && $row[$GLOBALS['TCA'][$table]['ctrl']['languageField']] > 0) {
    $PA['fieldConf']['config']['readOnly'] = TRUE;
    $PA['itemFormElValue'] = $this->defaultLanguageData[$table . ':' . $row['uid']][$field];
}

The IF-Condition is wrong (also true for translated independent content: $row[$GLOBALS['TCA'][$table]['ctrl']['languageField']] > 0).
In 7.6 in class SingleFieldContainer this condition has been replaced by a variable $isOverlay which defaults to false and is set to true:

// This field decides whether the current record is an overlay (as opposed to being a standalone record)
// Based on this decision we need to trigger field exclusion or special rendering (like readOnly)
if (isset($this->data['processedTca']['ctrl']['transOrigPointerField'])
    && is_array($this->data['processedTca']['columns'][$this->data['processedTca']['ctrl']['transOrigPointerField']])
    && is_array($row[$this->data['processedTca']['ctrl']['transOrigPointerField']])
    && $row[$this->data['processedTca']['ctrl']['transOrigPointerField']][0] > 0
) {
    $isOverlay = true;
}

So, a better way (by transOrigPointerField) is used to detect (dependent/independent) translations.