Britney Spears has sold too much and has too successful not to be considered a legend. If you like her music or or not you can't deny her success. She may not so as well these days but that happens to all pop stars eventually.
Do I favorably consider them legends? No. But are they, in a sense, legends of pop? Yes. Those names will forever be associated with the music genre and the era in which they rose to fame. Whether you are a fan or not, they were once at the top of their game, and no one but those at their level will be remembered in such article lists.
I love Britney, but I would think about her as the "pop princess".. a legend could be madonna and the amazing Michael Jackson, the backstreet boys are okay, but I wouldn't think about a boy band as a legend.