This is more or less standard for slim LED side-lit LCDs.
The light sources are at the sides (and sometimes the top and bottom) of the screen, and light is scattered across the panel by light guide plates. Apparently, this process hasn't been perfected. We've seen similar uniformity problems on nearly every LED LCD TV we've reviewed, and like you say, we think it's a pretty glaring problem on expensive panels. It's one of the main reasons I usually recommend Plasma displays instead, although they're not without their faults either.