Because if you know anything about my thoughts regarding Shanks, he is a purposeful walking contradiction. I am not saying he's siding with the WG, but I am saying he has ulterior motives with Luffy. Which IS true regardless of how you look at what hes doing (good or bad). Oda has confirmed he has his own plans for the new era, which implies they are not Luffy's plans.
My point is, out everything he DOES do publically, he has this very oddly casual relationship with the world's leaders, and that by default is extremely weird.