People do this kind of stuff for academic reasons, to figure out what's involved, the processes, the physical limitations or various items, the amount of time and skill involved, etc.
That's how they get away with being able to have academic discussions about building nuclear bombs. They're not encouraging anyone to do anything illegal, they're having spirited academic discussions (like: "no, you dumbass, if you do that, you'll blow yourself up and a hundred blocks around you will become a smoking crater!").
I mean, look at some of the stuff they show in the movies - ideas, techniques, etc. Think about the movie "Heat" and the Hollywood shootout. Think about the Clancy book,
Debt of Honor, and the subsequent flying of airliners into the towers on 9/11. That book came out in 1994, and when I saw the towers burning on 9/11, I IMMEDIATELY thought of that book (I read it a couple of years before).
You can't stop people from having ideas, and sometimes using other people's ideas to do something illegal, immoral, or deadly with them. But we CERTAINLY don't want to be the source of anyone doing something like that.
I have no idea how Clancy feels about his book and 9/11 - and of course they could have thought of the airliner thing completely separately, or how Michael Mann feels about the Hollywood shootout. Sure he didn't put the guns in their hands or told them to do it and he's not responsible for their actions, but IIRC, they got the idea for that bank job from the movie.
Freedom isn't free, and the exercise of that freedom comes with responsibilities, otherwise we'd have anarchy.