That is a tough situation. The bottom line is you really have just two options: remain an authorized user on the account or get taken off of it.
In my opinion it would be worth calling in a few more times and seeing if you can remove yourself. The customer service for store cards doesn't exactly have a glowing reputation, so it is possible you just got someone that doesn't know what they are doing. This might not get you anywhere, but I think it is worth a couple of calls.
It doesn't make sense, but the issuer has told you your consent doesn't matter in being an AU so you might have to get your Mom to remove you if you feel that is best. You will have to guess what the risk is of staying on as an AU and weigh that against the importance of sparing your mother's feelings. Do you have any idea how responsibly she will manage these cards? If you are reasonably confident she will always pay them you really don't have much worry about. If, however, you are not sure or have reason to think she won't manage them responsibly then you may determine it is in your best interest to have yourself removed even if it means having a difficult conversation with her.
If it comes to this I don't know what to say other than prepare yourself and approach it as tactfully as you can. Make sure she did add you as an AU. Thank her for the consideration if she was just trying to help your credit out, but explain since you have no use for them you would rather not have these cards reported on you. I would approach it as simply and straightforward as you can, not going in defensively or saying "don't take this the wrong way" - if she is very sensitive it may trigger her to wonder why she should in the first place.
Good luck! I will also refer you to a previous blog post
about authorized users, just in case you would like some basic information on the subject.