If it was an honest mistake made by the magazine then yes it is right for them to own up to it. If it is public knowledge that there was a mistake made then that needs to be corrected in writing in their magazine. It doesn't have to be big but they should acknowledge it.
Yes, the Rolling Stones magazine should acknowledge the mistake in the image of the United States Constitution rendered over Julia Louis-Dreyfus' back on the cover of the May 2014 issue, because with as much information as we have in today's age, when a person or publisher makes a mistake they should own up to it. They cannot sweep it under the rug.
Rolling Stones should acknowledge the mistake because it is the morally correct thing to do. By acknowledging the mistake, Rolling Stones is amending a wrong and therefore bettering their ethical standing. Acknowledging their mistake also means that Julia Louis-Dreyfus can save her reputation and social standing within the American societal hierarchy.
Rolling Stones magazine is not a political magazine in any way. I do not think they should take the time to note the mistake in the background. This seems like a lot of useless "mistake" finding, given that Rolling Stones generally has more to do with music. They aren't seeking to be factual in ever photo they produce.