Frankly, the very idea of a crime with no mens rea seems like a slap in the face to justice.
They make sense in some cases, to encourage entities to fully understand the law, but I think that much of that can be subsumed if we incorporated “willful/knowing/negligent ignorance of the law” as part of the mens rea of the root crime. In other words, how reasonable is it for said entity not to have known the law in question? For large and mid-size corporations with substantial legal staff, the answer should be, “not at all”. But for a kid not to know that state law requires any product labeled “Lemonade” to have a minimum lemon juice content, the court’s response should be, “Kid, go home. Officer, I want to see you in my chambers after session to discuss how not to waste the court’s time.”
That’s not what mens rea means. A law without a mens rea requirement means you get punished whether the act you took was intentional or not, not whether or not you knew the act was illegal. Not knowing the law will never protect you, mens rea or no. If the feather law had a mens rea requirement, it would mean that someone who picked up garbage at the beach, unaware that one of the beer cans had a seagull feather stuck inside it, wouldn’t go to jail for possessing the feather if some cop decided to search their bags on the way to the trash. If the law had a mens rea requirement, the cop would have to prove that they knew the feather was there and meant to pick it up. Whether the person knew they weren’t supposed to have the feather is irrelevant in either case, and no amount of lawyers will contribute.