From having worked in a front end type job in my youth, and equally well being on the other side obviously. I have to say that it's usually the d-bag on the other side of the counter that is being a d-bag and that behavior being mirrored back at them. I can't think of a single time a cashier ill-treated me for no reason. If you are getting this often enough to post on a forum about it, take a look in the mirror bud.
JM2C
JM2C