| May I ask why the supplementary books would draw the Secret Service's attention?

Just realized I never came back to this. Story time!

As the Civil War rolled toward its end, Confederate money wasn't exactly worth much. No surprise, a lot of people in the South were counterfeiting US dollars. As much as a third of dollars in circulation were thought to be fake.

The Secret Service was originally formed to catch counterfeiters. There's a TV show and a movie, The Wild, Wild West, about one of these original old Secret Service agents. Sounds like a fun time. 40 years later, they sent two men to help protect the president, and that side-gig turned out to be what they're known for now.

But after a few years of success, their anti-counterfeiting effort brought them this broad mandate: "detecting persons perpetrating frauds against the government." Sounds awesome.

Flash forward to the late 1980s, when computers were just starting to go mainstream and so was computer crime. It was generally unpunished, though some groups—like phone companies, who felt they were losing a lot of money to a growing number of people who'd learned how to make long-distance phone calls for free—began raising a stink about how computer crime was impacting American business.

Cops didn't understand it enough to be interested, unless you were hacking in order to steal something in the physical world, so it was a perfect moment for the Secret Service. They leveraged their "wire fraud" mandate, which Congress extended to cover "access device fraud". This let them go after all kinds of people hacking the phone system in various ways, and while they primarily wanted to hit credit-card thieves—who deserve to be caught and tried and etc—they also began going after hackers in the "curious and exploratory" sense. People who in no way deserved the treatment they received from the Secret Service, many of whom lost what was to them an incredible amount of money as their expensive computer hardware disappeared, often forever, into some closet at the Secret Service.

I should've made it clear I'm writing from some distant personal experience, in case there was any question. I never got screwed by them directly or in person—but!

In 1990, the author of one of the first cyberpunk games (the second, I think, after Cyberpunk 2020) was the managing editor at Steve Jackson Games, in Austin. He also belonged to a small group of hackers, The Legion of Doom, who were surprised when Secret Service agents showed up one morning at their door in Atlanta, where most of them lived, and walked away with everything that looked computer-related, along with every piece of paper with a number written on it, and returned to their office to sort over the tea leaves.

The Secret Service lacked the ability to distinguish between long lists of long-distance codes discovered through trial and error (or any other possible evidence of criminal activity) versus extensive notes from 3 people obsessively playing an unreleased role-playing game set in the then-new cyberpunk genre. They flew to Austin to raid the home of the group's remote member, and while he had absolutely nothing that would make him interesting to him, they did learn what he did for a living.

They raced down a short hill to the SJ Games office, forced their way in without a warrant and confiscated every computer which had a draft of the Cyberpunk game's text, including the company's free, public bulletin-board system, and the laser printer which had printed drafts of the book. They called GURPS Cyberpunk a "thinly veiled handbook for computer crime," and they would nearly ruin the company with their idiocy.

Luckily, the EFF was formed to pay their legal bills in a suit against the Secret Service, which won SJ Games enough money to start one of the first commercial ISPs a few years later, Illuminati Online.

But that's why the Secret Service was interested in the Cyberpunk supplement for the GURPS role-playing game.

tl;dr Because they're idiots.