They didn't recognize the most powerful person in television so they treated her the way they probably normally treat their minority customers. These boutiques have an etiquette all their own and usually includes complete indifference if not outright rudeness by the sales people.
Driving while black and shopping while black can be frustrating and rage inducing experiences if you've ever had to do it. My experiences have ranged from being followed around a store so closely I could feel the security guards breath on my neck to having merchandise ripped out of my hands. It seems that being one of the richest women in the world cannot cure these ills. Oprah's friend Gayle King who accompanied her on the shopping trip told Entertainment Tonight that "it was one of the most humiliating experiences of her life".
African Americans spend an average of $300 billion on goods and services every year. This fact has yet to be reflected in the way we are treated when shopping for those goods. Thankfully there has been some progress, advertisers have tried to tap into that market. Its just so sad that we still have such a long way to go.