There are two factors that come into play here in terms of regulations.
The first, and biggest, is the International Traffic in Arms Regulations (ITAR). ITAR is the set of United States regulations that control the manufacture, sale, and distribution of defense and space-related articles and services as defined in the United States Munitions List (USML), https://www.ecfr.gov/current/title-22/chapter-I/subchapter-M.... The USML is large, and broad in scope. It also still needs to be interpreted. These interpretations err on the side of 'an abundance of caution', due to the fines from violating regulations and the perceived threat to national security. ITAR regulations and policies are complex enough that most defense contracting firms have people who specialize in just ITAR and help others in the company deal with it. I have worked projects with multiple countries and can tell you that it is a pain to dot all the Is and cross all the Ts, even when working with countries that are our closest allies. Things that you might not think would be covered often are, or are interpreted to be covered. Let's say you designed a new type of paper clip. This paper clip[ is so much better that when it comes to organizing files and paperwork processing productivity its use is considered to provide a 'material advantage' to military units that use it. That likely would be export controlled under ITAR. I'm not debating whether it should or not, that is how it is.
The other factor is that modern conflict is multi-domain, and one of the biggest domains is economics. Corporate espionage is very real, and very state sponsored, around the world. That means that things like chip designs are very sought after.
I remember going to a particular standards conference. We had a huge upsurge in one country's delegates just before the conference (roughly a 3:1 increase), and during the conference I noticed the new delegates seemed more polished and less proficient than the ones I had worked with for a while and knew. The ones I knew also seemed deferential to the new delegates. Before the conference the matter came up with my employer. I've been told such situations are the norm rather than the exception.
It is because of these situations and the level of state actor activity, that the U.S. wants to know all your affiliations, even seemingly innocent ones. You sign a legal document, in effect testifying, that the associations you documented are accurate and complete to you knowledge. The penalties for not disclosing such associations are well known because they are spelled out. They are draconian because of the U.S. concern, and the causes of that concern are real. All sorts of cases can be made for that policy, and against it, but there is no away any researcher working for the government on DOD or DOD-tangential projects is ignorant of the policy.
As an anecdote, I knew another American colleague. Young guy, had a girlfriend in Canada (a relationship he disclosed). They decided to get married, and did (something he did not disclose). When the CO of the group he was doing his research for found out about it my colleague's employment and clearance were terminated, and he was judged a permanent security risk, meaning he could never again work a government contract - effectively ending his career.