That's not exactly what's going on in this thought experiment. If a crime is committed without mens rea, then it's still something bad that happened, like if a person kills another person but it was due to negligence rather than out of malice (and thus might not be murder legally). We don't think of that idea as "thoughtcrime," because the act itself (homicide) is fairly universally recognized as a crime (and a "bad thing") regardless of the thoughts behind it. But with intellectual property, that isn't the case. Most people would not consider publishing the nth prime to be a crime or a bad thing, but the law will treat it as a crime if the publisher happens to know that the nth prime is useful for breaking encryption (even if it's just to decrypt media you have bought legally). That's why, at least to me, negligent homicide doesn't feel like thoughtcrime, but intellectual property violation does.
I agree it's not precisely mens rea; my point was simply that criminality has always depended in part on what was going on in people's heads.
Note that it's not just IP. Imagine constructing a prime from which one could decode child pornography. For that matter, imagine finding a prime whose ascii representation was a death threat: publishing that would be legal; sending it to someone with the expectation that they read it as ascii would be illegal.
The question is what is really going on, and a part of that is what's in people's heads. Law isn't voodoo, it's how we regulate actual behavior of actual people.
"Thought crime", in its Orwellian context, is entirely different. I can think whatever thoughts I want opposing the copyright monopoly or even opposing prohibition of child pornography - in fact, in some scopes, I've thought both of these kinds of thoughts - and not be guilty of any crime.
