Marketing and market research person here.
The sales clerk wasn't very good at public relations, and could have worded it differently, but she was giving a sort of mangled version of the truth.
Shops do sell based on target demographics, and clothing in higher end shops tend to run smaller than clothing in lower end shops.
Race and social class are also demographics that are used in marketing.
This is all based on statistics/generalisations, so there will be many people who don't fit into the definition of what a black person, plus-size person, etc. is like, but targeting is really the only way that businesses can stay in business and make a profit. In other words, they can only afford a limited amount of inventory, so they have to make sure that the majority of people who come into their shops are going to buy the majority of clothing that they sell. This means that unfortunately, if you don't fit their definitions, then you will have to go somewhere else.
I don't know anything about the quality of Macy's market research - they could be basing their marketing on statistics that are out of date, or their researchers may just not be very good at analysing statistics properly.
As one of the commenters said, the sales clerk should have just apologised for the shop not having anything in her size, told her which Macy's had a wider selection of sizes, and then kept her mouth shut.
ETA: I didn't realise that there were so many pages of comments before; I only read the first page. Mort, I don't understand what is upsetting you about the comments.