The Uncarrier has caused a wireless revolution and shaken things up in that industry. Who else but legendary mover and shaker Shakira could star in an advertisement for them?
She already had a deal with T-mobile earlier this year. They sent a mass-text to all T-mobile users with a mobile link to download CRTFY for free when it was released.
So I can understand why she'd do a commercial with them, probably a partnership deal I guess.