For extremely wide screens it obviously looks awkward, but the idea seems sound in principle.
IIRC, the reason you'd want to restrict width of content is that it's hard for your eyes to track back all the way left, to the start of the content, when you need to go down a line. But the header is just a single line, so it doesn't have this problem.
In the case of very wide screens they should probably restrict the header width too, just not quite as much.