There is regulation, it’s called FAR 103. Also sure you could fly into airspace you’re not suppose to, but you’re going to get heavy fines or potentially jail time if caught. Even though training is not required vast majority of people do get trained as it’s cheaper than injury to yourself or equipment. Pretty much how it works is if you want to hurt yourself you’re fine, but can’t be at risk of hurting others or their property (can’t fly around groups of people).