Why are cashiers mean?

I mean seriously, why do they have to give you an attitude when you plop down your coupons? Yesterday I went to Meijers for a few things, and you would've thought that I had robbed the place, by the way the cashier was acting!
She took the coupons and one by one, started grabbing stuff off of the conveyor, to match the coupon, and would THEN scan it.
Seriously.
All you have to do is scan all of the merchandise, and then scan the coupon, and if the coupon doesn't match, then it won't let you use it.
But, to act like I've just stole stuff, is retarded!
I think I may have to go through my coupons, get a bunch of stuff, and find her isle again!