Substitute "Homebuilt PC without DRM" for AK-47, pretend it was written in 2023, not 2013, and then reread the article as well as the comments at Hacker News.
It also opened my eyes to the simple arguments "I like shooting guns" and "it's not the government's business", which I had dismissed so easily it's scary. I like to think the experience improved my ability to seriously consider opposing arguments.
EDIT: Obviously the problem is with circumventing the DRM - but in the context of the post I'm responding to (people home building machines with no DRM) I thought I was obvious.
Homebuilt PCs are completely legal everywhere I know. Not including any DRM-related software or hardware does not constitute a crime, as PC has no parts that are mandatory by law.
On the contrary, homebuilt firearms are illegal to create and/or possess in some jurisdictions (but completely legal in others, laws on this matter vary from country to country).
"Homebuilt PC without DRM" [...] pretend it was written in 2023
The implication is that the machine is built to circumvent DRM, not just to avoid using any DRMd media. Obviously avoiding DRMd media is legal. But using something to circumvent DRM is illegal.
To directly answer your question: Licensed software and permission from rights holders.
There's nothing illegal about not having licensed software on your computer.
Here's the anti circumvention article (https://en.wikipedia.org/wiki/Anti-circumvention)
The problem is in circumventing the DRM, not just having a machine with no DRM on it. But that's the implication in a dystopian future-story where people home build machines with no DRM.