This is how it has always functioned since it was released in SW2011 SP0.
I have actually used it on solid parts, as it can be used to make raised panels and other bump-outs easily. Very counter-intuitive though.
I don't think you are "missing anything", that's just the way it is. I do not know why.
Thanks for the reply Charles. I wasn't sure if I was doing something wrong. Honestly, I think it should be considered a broken feature. If I wanted a solid feature I would have chosen so. Would be nice if there was a check box to make it a solid or just keep it a surface if I capped both ends as well as capping both ends of a single direction extrude.
This is by-design since it is a surface feature and those who asked for capped ends on the extrude surface want it to remain a surface until they make it solid (thicken), subtract it from a solid or trim it with another surface.
I'm a little confused by your reply, I'm sorry. The way I understand your reply is the exact way I would expect and want. My issue is that the surface extrude feature is making it a solid without me thickening (with the thicken feature) the closed (capped both ends) surface. If I cap one end, it remains a surface of course, but capping both ends makes it solid. I need it to remain a closed surface because I need to trim and knit with other surfaces.