301 Redirect on uncharacterized articles

I've noticed that my uncharacterized articles are showing a 301 redirect in the server headers. Is there a reason behind this? Some article sites have rejected links to those pages since they are "redirected".

Is this an SEF Advance feature or something on my server?

Probably something specific for your site.

Here is a uncategorized article URL: joomla15.sakic.net/example-pages-and-menu-links/
Check its headers with some tool like www.seoconsultants.com/tools/headers.asp

#1 Server Response: joomla15.sakic.net/example-pages-and-menu-links/
HTTP Status Code: HTTP/1.1 200 OK

What's the URL you are checking? Note that if you click on a link without trailing slash (like joomla15.sakic.net/example-pages-and-menu-links ) it will first redirect to a link with trailing slash and then return 200 and that's a SEF Advance feature.

If I use the trailing /, I don't get the 301. The page is www.everythingpre.com/pre-accessories

Is this a default feature or something that is configurable on the sef advance control panel?

It's a default feature to imitate behaviour of a web server directory and create appearance of physical directories. For example, if you click on www.joomla.org/images which is a physical directory, Apache will redirect you to www.joomla.org/images/ with 301.
SEF Advance works the same way.