Let's face it, cliches aside, we all know a cover is a sort of promise. There's a language in covers that develops telling us that a clutched couple means this, a certain color scheme or lighting means this.
Now, yes, most readers have come to learn it is an inexact promise. That sometimes the chick on the cover has long blonde hair only to discover that the girl inside has short red hair. It can be little like following a promising headline, only to discover that the actual story is not nearly so interesting. And while certainly a cover is just part of the package of things hinting at what's to come, it is still part of it.
So - what happens if it turns out the girl or guy on the cover is not even the same race as the book character it is supposed to represent?
I know in this post-racial world such things aren't supposed to matter. Except that they do. First, let's face it, I have not heard of a book where a white character was represented on the cover by a non-white character. Second, there are a lot of people trying to find books that are about a range of people, if even the cover doesn't accurately represent the book, how can they find such books outside of the African American reading section (which is not only a misleading section name, but also jumbles together a number of genres, and, of course, still leaves no clear place to find books about other less well represented in mainstream fiction groups such as Asian, Hispanic, and Indian).
And here's the other reason, some people don't want to read about black (or blue or purple) characters. Alison Kent had a book featuring an interracial romance inside. The cover, sadly, didn't reflect this. Now sure, you could argue that maybe someone who wouldn't have otherwise picked it up did. Again, I will ask, if that's so, how come they never try it the other way around?
What fascinates me is that in other mediums, it is agreed that such deception is wrong. Music customers who had purchased Milli Vanilli were offered refunds after it was revealed that the people purported to be doing the singing were not. Now, that was more an issue of perceived appeal, than race, but the idea was similar that it was assumed that consumers would prefer a certain look even though the product itself was primarily aural. (I recognize that look is still a factor in music, but then so are covers in books,no matter what your parents might have told you.)
So, to circle back to my original point, the cover is a promise. Yes, it's marketing, but that marketing is supposed to be indicative of the product. Using cover models of the incorrect race violates that promise indicating that the publisher either doesn't care enough to understand basic facts about the book or suggesting willful deceit. I imagine the reality is likely more complex and less sinister, but when a reader opens the book and discovers the issue (or possibly doesn't even open the book) they will be upset. They may still love the book, but will also spend time and energy wondering why the cover misrepresented the contents.