IUSE="gudev hwdb introspection keymap selinux static-libs" DEPEND="" RDEPEND="|| ( ~sys-fs/udev-171[gudev?,hwdb?,introspection?,keymap?,selinux?] ~sys-fs/eudev-0[gudev?,hwdb?,introspection?,keymap?,selinux?] )" static-libs doesn't force anything here.
right, udev-171 always builds static-libs but newer udev's doesn't. so it's required also in the older ones so apps can add dep like: virtual/udev[static-libs]
USE=kmod is also useless flag for 171 but it's there to allow virtual/udev[kmod] dep to be used this will autoresolve itself after removing 171 from tree after the stabilization of 197-r3 is done