Common misconception... there is no law or rule about flying hobby drones above 400 feet. It is a suggestion. Not a rule or restriction.
Oh, so if I'm a hobbyist, i can fly above populated areas, over an airport (heck, right over the runway), 20,000 feet up, right beside a commercial jet? Wow, why would anyone want to get certified if there are NO RULES for those who simply remain ignorant? No, sorry, that's not how it works.
The guidelines are a starting point while the laws are being written. If you ignore them, you risk even stricter laws coming to fruition. The guidelines are a looser interpretation of the regulations commercial pilots need to follow. The difference being that a commercial pilot has the information they need to fly where they need to with proper permission and/or waivers. The hobbiests isn't expected to know anything so the general guidelines, which you agree to online (as in, it's a legal contract at that point), if you register, are there to keep them out of trouble. If they are disobeyed, the registration number provides identification so proper actions can be taken, whatever that may be. There's no jail time associated with the infractions, yet, but if your drone kills someone or causes property damage, you WILL be responsible. Actually, that's regardless of following and safety regulations, but the regulations are there to help avoid those situations.
Be smart people. Use common sense. We've all seen, in these forums, people who go against the guidelines and cause personal injury, property damage, or lose/crash their drone. The point is, if you follow the guidelines, all of this can be avoided. Law or no law, i know we're asking a lot, but please use common sense.