This article left out a lot of details, but to me it sounds like what happened is that the FBI infiltrated PlayPen under a warrant granting them access to do so. They then used PlayPen's tor node to trace inbound traffic across the tor network and identify the IP addresses of visitors.
I could be wrong, but if that's the case, it sounds to me like the defendant didn't have a reasonable expectation of privacy. I think of it as sending a letter with no return address. If the letter is addressed to a criminal enterprise, and there is a reasonable expectation that the sender is engaged with said criminal enterprise, to the extent that the FBI can trace that letter back to the sender seems that it would be legal, in my opinion.
However, the TL;DR of this article seems to be that nobody's computer can be expected to be private because everyone's computer can be hacked. I don't think that's what the judge intended with this decision.
I could be wrong, but if that's the case, it sounds to me like the defendant didn't have a reasonable expectation of privacy. I think of it as sending a letter with no return address. If the letter is addressed to a criminal enterprise, and there is a reasonable expectation that the sender is engaged with said criminal enterprise, to the extent that the FBI can trace that letter back to the sender seems that it would be legal, in my opinion.
However, the TL;DR of this article seems to be that nobody's computer can be expected to be private because everyone's computer can be hacked. I don't think that's what the judge intended with this decision.