The wikipedia write-up suggests other grounds. From the majority opinion (via wikipedia):

> The evidence that petitioner was ready and willing to commit the offense came only after the Government had devoted 2½ years to convincing him that he had or should have the right to engage in the very behavior proscribed by law.

Jacobson was let off on grounds of entrapment; the reasoning was that the government specifically persuaded him to do what he otherwise wouldn't have done (or at least, what it could not be reasonably shown that he otherwise would have done). It's not obvious that that has much to do with whether he was or wasn't aware that child porn was illegal; the fact that the entrapment campaign included political literature protesting government intrusion tends (IMO) to suggest that he was.

Legality does get coverage as an issue, but it seems to be mostly (again, all I did was read the wiki article) as a way to show that Jacobson wasn't a suitable target for the entrapment campaign in the first place; the argument went that buying legal child porn doesn't demonstrate that he's likely to buy the same child porn after it becomes illegal.