They're glued in the 2013 retina models also (I know -- I've replaced the battery twice). To do it "properly" you pretty much have to take apart the entire thing, remove the logic board, etc., although if you're willing to cut corners you can sneak it out without too much fuss. In theory you can use a light amount of acetone to loosen the glue, but that always made me incredibly nervous so I just prodded it with a plastic spudger until I could get it free.

I would gladly trade a fancy unibody for a battery that's easily replaceable :-/