Er, whyever wouldn't it? On Linux it certainly works as expected, and it seems that it works fine on OS X when avoiding the broken Go shims.
A bit like the .signature named pipe from fortune thing, where sometimes you got two signatures and sometimes none? Anybody remember that?
signature file can't be a named pipe (or /proc/uptime) https://bugzilla.mozilla.org/show_bug.cgi?id=62993
can't attach signature from pipe (fifo) https://bugzilla.mozilla.org/show_bug.cgi?id=163831