fill on "piece" breaks the hierarchy

[mdsumner]

ggplot(holey) + aes(x = x_, y = y_, group = branch_, fill = branch_) + geom_polypath()

That results in the windows being filled, as per fill = branch_ - but this means the fill aesthetic has broken the group aesthetic into multiple calls to pathGrob() - is this inherently the way the world has to work?