Well, I hate to spoil the fun, but I have diabetes. That means that my vision can be quite blurred. Not always, only when there is too much sugar in my eyes.
When that happens, I may be staring right at you, and only see a ghostly silhouette. I will see that you are there. I will be able to look you straight in the eye, simply because I know where eyes are at the human body, not because I can actually see your eyes. But I will not see the color of your clothes. I will not see your name tag, or that you even have one. And you will not see that there is anything wrong with me because I am not blind, only my vision is blurred.
Under those conditions, I might ask you whether you work there, simply because I do not want to bother other customers. I might also ask you to point me to some item I am actually standing in front of but cannot recognize.
I am sorry if that bothers you, but if you work at a store, isn't it a part of your job to assist the customers? Aren't they the people who ultimately sign your paycheck? You don't know what reasons they have to ask you something that is obvious to you. You don't know they may not see you clearly. You don't know they may have just lost a beloved person and their mind wanders on other things than your name tag or the fact you are standing at the cash register. They are most likely trying to be kind because they do not want to bother you if you do not work there. Besides, if you snap at them, they may never come back, and you may keep wondering why the store suddenly cannot afford all of its employees and why it is you they decided to lay off.
Even if you are right, and the person asking you is a jerk who should know better, even then he is the jerk who pays for your groceries and for your Internet access.