The simple act of console hacking is only illegal if you do it to someone else's console without their permission. Granted there's the whole end user license agreement where you agree not to do it, and it is a legally binding contract, but breaking a contract isn't exactly illegal, it just means you can't sue when you get banned for hacking the console.
If it were to be illegal, it would likely fall under trespassing laws and here's a summary how it would go down in court.
Court: "You are accused of the crime of trespassing by circumventing security measures in an electronic device. Do you understand the charges?"
Your Lawyer: "My client does not understand the charges as the electronic device in question belongs to my client. How can one trespass on one's own property?"
Judge: "Prosecutor, is this true? Is the electronic device the defendant is charged with trespassing on the property of the defendant?"
Prosecutor: "Yes your honor, but the security measures were installed by the factory prior to the defendant coming into possession of it, and there is a legally binding agreement where by the defendant is not to trespass."
Judge: "If the device itself belongs to the defendant, then the defendant cannot trespass on their own property. Violation of agreements is not a crime and is a matter for the civil courts, not criminal courts. Case dismissed."
Now, if you go DISTRIBUTING pirated games. That's a whole 'nother matter. That IS illegal. But simply hacking a console you own to run software you want it to run... no... that's not actually illegal, it's just violating the EULA. Basically, Nintendo can sue you, they can ban you, but they can't press charges.
But what I'm asking is is developping exploit and releasing hack to public legal?
Depends on the country. The guy who developed DeCSS was sued for quite a lot. DeCSS is what allowed for third parties to produce DVD players to play DVD movies without licensing the decryption key from the people who owned the patents on DVD movie technology. The court found that he only trespassed on his own equipment and could not be penalized for that, but distributing the encryption key was not allowed... in his country... In America however, it was a different story. There were zero attempts to silence neither the encryption key nor the algorithm because it wouldn't hold up in court. Interoperability was at the time quite legal in America.
These days we have the DMCA and things are a tad more hairy.
As for distributing exploits for this stuff... technically legal as long as it doesn't contain any code or content written by Nintendo. This is why a lot of homebrew apps for 3DS display a Homebrew splash screen instead of the Nintendo 3DS splashscreen. Because the splash screen is in the app itself, not the firmware. This is how Freeshop was initially crushed, it used the 3DS splash screen. Now it uses Homebrew and there's nothing that Nintendo can do about it.
Basically since you can't trespass on your own system, the only thing you have to worry about, at least in America, is "am I distributing code, images, sounds, or anything that was made by Nintendo?" if not, then you're fine.
Some countries are even more permissable than America. I don't know how true this is, but I've heard that in Australia, the laws actually promote reverse engineering for the purpose of interoperability, which is precisely what homebrew is.
Also, I am not a lawyer so... probably best to consult a real lawyer. I'm just well versed in the law, but no where near as much as a real lawyer.
Is it like not legal, but not illegal as well?
This much I am 100% certain of. Nothing is legal, only illegal. In the absence of a law preventing something, that thing is not illegal. There is no law saying that it's legal for you to breathe. There is no law saying it's legal for you to eat. There is no law saying it is legal for you to exist, see, hear, or beat your heart. This is a legal principle called "
Nulla poena sine lege" in lawyer speak, which means "no penalty
without a law"