power?
What strikes me most odd about this is the term "powerful". Steven Speilberg has "power", Oprah has "power", damn, i would say even the Olsen twins have more "power" than Britney (and a hell of a lot more money).
U2 has power because of their longevity, their proven record of being one of, if not THE, biggest album selling global band, their proven record of being one of, if not THE biggest ticket selling band, and throw in with that their political slant, and you you have a powerful entertainment entity.
This past year Britney has had a record which performed well below expectations, she had a movie that was a commercial dissapointment, her tour is playing to mixed reviews and mixed ticket sales, and she really has no clout outside the teen industry.
I just don't see the "power" that other people on this list have.