Just because they won’t use it, doesn’t mean someone else won’t find it and use it.
A backdoor is a built-in vulnerability and can be used by anyone. You should explain that doing something like this is very risky for your company. What happens when some malicious attacker finds this backdoor and uses it? This will cost your company a lot of time and money to fix. And what will your company say when people ask why the software contained that backdoor in the first place? The company’s reputation might be damaged forever.
The risk is certainly not worth having it in the code.
If you’ve informed decision-makers and they’ve decided not to do anything about it, then by definition your company is knowingly shipping a product with a serious security vulnerability. (And, I assume, hiding it from their customers.) This is a very serious matter. What’s the worst that a malicious person with access to this backdoor could do? If it’s bad enough, I would go to the FBI about it. (Or whoever has jurisdiction over computer security if you’re not in the US.)
If your company knows about the problem and doesn’t care, then exposing it is the only ethical course of action. And if they attempt to take retaliatory action against you, you may have legal recourses available, depending on the circumstances and the laws where you live. (Talk to a lawyer about that if you think it might apply in your case.)
Please, pardon my cynicism, but this isn’t the first and won’t be the last backdoor we see in our legitimate, hardly-earned apps and devices. Just to refresh our memory, we can start from the most recent one, the new Amazon’s Big Brother Kindle .
But we have an entire plethora of backdoored software and services, such as PGP Disk Encryption , ProFTPD  or Hushmail , to name a few.
And don’t forget the OSes: M$ is always ahead with its NSA_KEY , but also OpenBSD  and the Linux kernel  can’t be considered 100% safe. We also have paid attempts to gain a backdoor access to Skype by NSA , that, however, has been assessed as “architecturally secure” .
Moving down to firmware, nowadays we are almost acclimatized in having people from our ISP that are able to watch inside our routers (yes, maybe even see our beloved WPA password), but these  can surely be considered as backdoors too!
Finally, a few considerations on hardware and BIOSes , and (this is both funny and somehow dramatic) EULAs , because also lawyers have their backdoors.
Ok, given this preamble, I’ll try to answer to the question briefly. No, you’re not wrong getting mad for this thing, but you should focus your anger on the correct motivation. You should be angry because you lost a piece of trust towards the company you work for, not for the fact of the backdoor itself (leave this anger to the customers).
And if I were you, I’ll just be very cautious. First, I’ll make really really sure that what I saw was a backdoor, I mean legally speaking. Second, I’ll try in any way to convince the company to remove the backdoor.
You probably signed a NDA  with your company so your question here could be already a violation. However I don’t know where the NDA ends and your state law begins (it could be even customer fraud), and probably, due to the technicality of the subject only a highly specialized lawyer could help you with this matter. So, if you want to proceed, before doing anything else, even talking to the authorities, you should hire a very skilled lawyer and be prepared to lose a lot of time and money, or even the job.