It is the second part of the sign that makes the act of posting it most likely illegal (at least in the UK) as incitement [1].
The analogy's main issue is that it fails to distinguish between theft and copyright infringement which is exactly what the anti-piracy bridge are often accused of doing. Also, a sign is a poor analogy as it is in passive view of the public. A better one (ignoring the theft/infringement issue) would be a guy sitting in a pub who would tell anyone who asked him which stores had no staff in, and that they could take what they wished.