Is it a G-bundle?

Question

Remark 3.3 in Behrend and Dhillon's paper "On the motivic class of the stack of bundles" says $\mu(BP) \mu(G) =\mu(G/P) $ for all parabolic subgroups of G, where this requires the torsor relations only for the group $G$. I wonder how is this proved, do we show that the natural map $G/P\rightarrow */P$ is a $G$-bundle? If yes, could you give me a few hints on how to show it's a $G$-bundle. Thank you for your time.