USE crypsetup does not change installed files anyhow, udisks uses cryptsetup even when built with USE="-crypsetup", no option related to it is in configure.ac, in Makefile* building src/udiskslinuxencrypted.c and src/udiskslinuxblock.c is not optional.
It's not automagic because the package doesn't link against to cryptsetup libraries which is why it's only in RDEPEND instead of both, DEPEND and RDEPEND. AFAIK the functionality is used only for cryptsetup managed disks/partitions Or provide a way to reproduce a real problem with the way the dependency is handled in the ebuild
Now metadata.xml has: <flag name='cryptsetup'>Enable <pkg>sys-fs/cryptsetup</pkg> support</flag> So with USE="-crypsetup" user expects support for cryptsetup be disabled, but it is not. More correct description would be something like "Depend on <pkg>sys-fs/cryptsetup</pkg> for supporting encrypted devices".
