[Sharply and without hesitation:] Because you are being a nuisance.

Customer or not, that does not allow you the right to cause a scene in an establishment, harass the worker, and hold up a line. You may have your say but you're having it rather loudly and rudely.

[As for why she won't leave, it's pretty much because she hates leaving things like this alone. It's quite akin to bullying and she's bad at holding her tongue. She doesn't even see it worth addressing.]