Having worked sales myself (and as a manager), I can tell by what she said that she just doesn't want to deal with a return. It's better to please the customer the first time, or do as they request as best as possible, than to do whatever they want to do. I would not call that normal behaviour even though you see it so much. If it really upsets you that much go back and return it then request to speak to a floor supervisor or manager.
I had a situation that really tweeked my nerves too the other week at Saks of all places. I was very interested in purchasing one of the Bond no.9 fragrences (not cheep even with a discount). Well when one of the ladies came over to help me out she didn't want to show me the Bond fragrances, she kept trying to push cheeper ones on me that I wasn't even expressing interest in and kept saying no to. I was very upset about that and I WORK there. It's just ignorance.