Agreed. I have the same laptop. There is supposed to be a BIOS update which allows it. I have not tried it I bought the smaller 45w HP adapter so I can be more mobile. It would be nice to be able to use a 3rd party port replicator with a power cable to charge the laptop.
I recently had difficulties connecting my Spectre 13 laptop (Kaby Lake) with a USB-2 keyboard dongle and printer using my third-party USB-C hub, so I went ahead and purchased the HP USB-C Dock Station (Product #Y0K80AA#ABA). It works! It both charges my laptop over USB-C and it solves all the connectivity problems I had with the USB keyboard dongle and printer. I should've just purchased the (more expensive) HP hub for my HP laptop to begin with.
HP released a new BIOS update last week, Version F.33 Rev.A, for my Spectre 13 (Kaby Lake) laptop. The only fixes listed are "Provides improved security of UEFI code." Just for grins, I got out my old third-party USB-C dock (Plugable) to see if the laptop still complains about an unauthorized charger and refuse to charge. Sure enough, no change and the Plugable USB-C dock still cannot charge my HP laptop.
Fortunately I've since switched to the official HP USB-C dock which works great and does successfully charge over the single USB-C cable (see my earlier post).