You just cited one of the stupidest legal fictions ever created. Yes, everyone knows it. Yes, it's been around since forever. And yes, it's ridiculous.
How many federal statutes are there? Trick question: no one even knows. You could spend your whole life reading the Federal Register and you still wouldn't know the whole law. And even if you did, there are statutes that incorporate the entirety of "foreign law" by reference ("No animal may be transported in violation of any state, federal, or foreign law."). So you'd need to memorize every law in the world.
There needs to be some sense to this imputation of knowledge. "I didn't know it was illegal to kill someone" is retarded; of course you did. "I didn't know it was illegal to break into that guy's house"; again, ridiculous.
"I didn't know that Honduras prohibited transporting lobsters in clear containers, rather than opaque ones." That's not at all ridiculous. And someone was convicted for that and sentenced to jail.
"Ignorance of the law is no excuse" comes from a time when mob justice was close to the only justice. "We all think you did something bad, so you must have known it was bad, too!" There are still many crimes that have the quality that "you must have known you were doing something wrong, even if you couldn't cite the statute".
But there are others that, while valid criminal laws, really should only be enforced against people in some profession or other. If you own a company that catches, kills, and sells for food various types of wildlife, you should know if the state you're hunting in adds a turtle to the protected species list.
If you're some restaurant owner halfway across the country, and you just bought a shipment of turtles for your turtle soup from some company you'd been doing business with for years ... you probably shouldn't be held liable. You would think, quite rightly, you didn't really have to worry about endangered species law since you're buying from a legit corporation, and you know that the species isn't endangered because it's one of the most common turtles in the country so you didn't think to check if Rhode Island had changed its law recently.
This happened, too: some kids lobbied the state government to make this common turtle the "state reptile", and the state did, and the state's laws said "all state animals are protected species", and federal law prohibits trafficking protected species across state lines, and some company was negligent, and some restaurant owner was unaware the company was negligent, and some federal prosecutor was a douchebag, and now this poor guy is a federal criminal for making turtle soup using a turtle species which isn't at all endangered and which isn't protected in his state, at the federal level, or in any state except one random state that thinks it's cute to let 4th graders write state laws . He went to jail because of a Rube Goldberg-esque legal dominoes game.
There are too many laws, and society is too complicated, for us to keep saying "ignorance of the law is no excuse". You're right, but you shouldn't be.