Most if not all soundbars are designed to simulate surround sound, so rear speakers aren't needed. Having rears, wireless or not, kind of defeats the purpose of a soundbar and makes them just an expensive front array that can be trumped in sound quality by a solid 3.1 system.
A Rocketfish wireless kit can be added to most standard systems and receivers to get a signal to the rears. It's $99 from Best Buy. There are also subwoofers with built-in wireless, but they're not cheap.
Don't believe everything on the Interwebz! A duck's quack DOES echo!