But for any serious charge you need indisputable evidence, and usually lots of it, usually the threshold is quite high.. But in the United States the system is insane so I could be wrong

All the evidence required is that it is there.

Does not matter if someone else put it there without your knowledge. Or maybe it matters, but then it's on you (for all intents and purposes) to prove that it was without your knowledges.

The punishment for strict liability is much lower than for the same act with intent.

It's still essentially a death sentence if you are innocent.

In criminal law you must be aware of an item for it to be under your possession. If someone plants illegal content on your computer, and you are unaware, you aren't legally in possession.

"Typically in criminal law, the defendant's awareness of what he is doing would not negate a strict liability mens rea (for example, being in possession of drugs will typically result in criminal liability, regardless of whether the defendant knows that he is in possession of the drugs)."

https://www.law.cornell.edu/wex/strict_liability

Note the word 'typically'.

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.