I am using Alfresco's web client (which I built myself from a recent svn snapshot) to add new content to the repository. The operation is not related to any coding on my part. In the web client I simply create new HTML content and click on the HTML button in the WYSIWYG editor, which opens the "HTML source editor" in a separate browser window. When I type in an opening and closing form element , select update, and then re-open this editor the form tags are gone.
I have subsequently found a work around by creating this content item as type TEXT instead of type HTML, but I am still interested in why these restrictions exist in the latest code base.