The purpose of strict liability in possession is to prevent the defense that someone does not know the legal status of an item in their possession. It does NOT prevent the defense that someone does not _know_ something to be in their possession.
For example, it is not a defense to have drugs and claim "but I didn't know they were illegal". It is a defense to claim "I did not know they were there."
In drug cases with actual possession, it is difficult to support a defense of "I didn't know they were there", which is why charges typically result in criminal liability. They drugs were physically on you, and unless you have evidence that someone planted them, it is unlikely you could establish reasonable doubt.
But in cases of electronic material for networked devices, there is most certainly an affirmative defense to counter actual possession and constructive possession. Computer devices are hacked all the time, and network & device logs exist. For example, if a prosecutor agrees to the fact that a defendant had no knowledge of the material, a judge would toss the case and a jury would not convict you. The law is not meant to pedantically convict you of non-crimes.