Basically, this is because the default page of the Collaboration Publishing Portal uses the Publishing Infrastructure and the page is associated to a Layout Page.

If you really need to edit it, you need to detach the page from the Layout Page. You can do this by right mouse clicking on the home page and then clicking Detach from Page Layout.

After you click to confirmation to detaching the page, all the mark-up of the page to the default.aspx file.

You must check the file out to complete the process.

You will then be able to edit the page as it it was any aspx page.

If you need to revert the page back to the layout page you can do this by right mouse clicking on on the default.aspx page and clicking Reattach to Page Layout.

Any changes to the layout of the pages will be lost, but changes to the webparts that exist in webpart zones of the layout page will be retained.

You can do this with any page that is based on the publishing pages framework.