This is my third rMBP and my other two didn't have this problem (which doesn't doesn't mean they wouldn't have eventually, mind you). When looking on ifixit's teardown, they make it sound impossibly hard to remove the battery. However, the adhesive on this one looks to be a strip of double-sided adhesive foam, which makes me wonder if they changed the design to make battery replacement easier for technicians?
Are you in the United States? If so you are covered by the Magnuson-Moss Warranty Act (if not, google the equivalent in your country). According to that act you are allowed to open and service some of the parts inside your machine (feel free to mention this act to the genius if need be, though don't do it pompously, just make it clear you know what you are talking about and won't be bullied). Obviously you can't mess with the batteries according to the MMWA because it is a biohazard to do so, which brings me to my point. If I were you I'd bring the unit in, with the picture, explain to them how you discovered this issue, and then claim you demand that they fix your machine because as it currently stands, it is a biohazard. The batteries shouldn't be rattling around like that and there is a genuine risk the batteries could puncture as a result of their movement when the machine heats up, which it will under normal working conditions. Make it clear that's not an acceptable risk that you are willing to take. Since you already went in for this problem previously, there is a record on file that this isn't your first complaint, and now you have evidence there is something severely wrong with the unit despite their claims to the contrary.
If they say they won't fix it, ask to speak to the store manager and demand a refund for your machine because you will not accept a machine in such a condition, you paid good money for this machine, you shouldn't have to live in fear of dealing with a battery leak, which isn't paranoia since your picture is evidence of this risk. Mention that this level of service is incredibly disappointing since Apple has some serious quality control issues. Mention this is your third unit and frankly your fed up of this mess. Say you won't leave until you have come to agreeable terms. Now, if they give you a new machine, which they should, ask to be compensated for your troubles. I complained about having to have my screen swapped twice and Apple gave me a card that's good for a 10% discount of my next purchase, whatever it is. If they continue to stonewall you, after giving them a chance to make it right (which I'm sure they will if you are persistent and courteous), mention, only as an absolute last resort, that you have no qualms of taking this issue to small claims court (which by the way you should do if it comes to it, though I doubt it will, with the picture I think they will likely now acknowledge the issue).
Most important things: #1 Do not get angry and emotional. #2 Address them with respect at all times. #3 Do not take no for an answer. #4 Try to schedule an appointment for a peak hour, so that many customers will hear you complaining. Apple cares about it's image and if there is someone complaining about the computers being a biohazard, and how there are all kinds of quality control issues, Apple will feel pressured to just get you out of the store happy. If you were frantic, they know other customers would write you off as a lunatic, but if you are presenting your case rationally and convincingly, the other customers will be taking notice, as will the managers.