One of the many perks of being a celebrity is that you can go by whatever name you damn well please. Demanding that others refer to you only as Snoop Dogg when your birth certificate says Calvin Cordozar Broadus, Jr., might be grounds for an HR violation if you work at a conservative paper-supply company, but it's a completely respectable alias for a legendary rapper. (It also helps to differentiate from all the other Calvins in the music biz, such as Calvin Harris, whose real name isn't Calvin at all but rather Adam Richard Wiles.)
Like Cher (born Cherilyn Sarkisian), Madonna, Beyoncé, and Sting (aka Gordon Matthew Thomas Sumner) before her, Robyn Fenty ditched her regular-person name for the mononymic Rihanna — her middle name, which comes from the Arabic word for "sweet basil" — when she first entered the public eye. But a stage name is a stage name: The Barbadian star is still Robyn to her friends and family, and she gives her non-musical projects, like her Fenty x Puma collab, the Fenty name. Hence, Fenty Beauty by Rihanna — an obvious choice for her instantly-iconic new beauty brand.
Apparently it was not so obvious for some of the singer's so-called fans, since a bunch of them just discovered several years too late that Fenty is not a random assortment of letters or some kind of acronym but actually Rihanna's real-life last name. For some reason, these people are broadcasting their dearth of knowledge on Twitter... and it is not going over well with the Rihanna Navy.
Do the fans at fault deserve to be publicly dragged? Perhaps not; we are all just trying our best in this life. But on the other hand, if you don't know that Rihanna's real name is Robyn Fenty — after all the interviews she's given about her persona vs. her real self, after reading her official bio on her official website, after even glancing at her Wikipedia page — you haven't really been paying attention. Did you also not understand the plot of Fight Club? Spoiler alert: The narrator and Tyler Durden were the same person the whole time.