The gun free school zone act has been upheld even though you could be within 1000 ft of one with no real indication there is one there. Supposedly you can only be convicted for doing it knowingly, but IIRC knowingly has been interpreted to mean as little as you live near the area so reasonably should have known.
Also note, i.e. stuff like statutory rape has been upheld even in cases where the perpetrator in all good faith thought the victim was 18+, the victim initiated selling the services, and the victim provided fake ID showing they were 18+.
So there is not necessarily any need for mens rea in the US legal system.