What's the fun of buying ready made parts and just assembling it? At least the authors probably learned something new in the process.
Besides, the fuel "hacking" they seem to be doing seems relatively simple, isn't it what is commonly called "Rocket Candy" or something like that? In that case, it's a fairly common propellant made by amateur's. We're on Hacker News after all, as long as they're not hurting other people, if it's illegal or not should matter less.
For the same reason that it's generally illegal to make your own fireworks: you have a pretty good chance of killing yourself or more importantly kill someone nearby.
The risk of letting random people blow themselves up is one thing, but having innocent bystanders get killed or maimed is totally different.
Most fully developed adults agree that putting yourself at risk in the pursuit of your hobbies is fine, with limits. Putting others in danger who are unaware of the risk to life and limb is not acceptable. You, a private citizen, simply do not have the right to produce bombs as a hobby because of the undue risk to everyone around you.
As long as they can prove that they are not risking hurting other people. Suppose that I fire a gun in a randomly chosen direction and don't hurt anyone, should I be allowed to repeat the process?
I don't know that my personal opinion is all that important but I fully admit to driving in excess of the speed limit in very specific places where I deem it safe and others do not. I don't know anything about shooting guns, unfortunately.
What's the fun of doing a bunch of work but not getting a successful launch (best case) or blowing your hand off or burning the house down (worst case)?
a) Very dangerous, if you don't know what you are doing.
b) Illegal without a licence in some countries (such as the UK).