My take is that watching unlicensed content via a pirate medium is illegal if the viewer knows or has reason to believe that the broadcaster is not licensed to show it. Whether it is prosecutable, is a different matter.
It depends on whether the reasonable-ness test is objective or subjective. An objective test is: "would a reasonable third-party person have known?", whereas a subjective test is: "should the defendant have reasonably known?".
The difference is subtle and significant. A reasonable third-party person might know that an illegal decoder is being used, but the defendant may be too ignorant or not be in a position to know that the decoder is illegal. Which is more just?