In Joomla! 1.5.8, a default filtering choice is in place for those who have not selected an Article Filter option. The default applied for those who have not made a selection implements "black list" filtering as a security precaution against possible XSS.
If the default filtering option is not suitable for your needs, you can change it by using the Article Global Configuration options. Instructions for this feature can be found by pressing the Help button when in the Article Manager, or by clicking this link.