It is required by law. Most likely through prohibitions for "exporting" defense technology, which can be very broad. "Exporting" can consist of having a casual conversation with a non-"US Person" or having a document visible on your desk.
"Secret" classification is really mild, about 5 million people have one.
>The law prohibits employers from hiring only U.S. citizens or lawful permanent residents unless required to do so by law, regulation or government contract.
At these workplaces it will be a combination of all 3 of law, regulation, and government contract.
You are referring to ITAR. Under ITAR, permanent residents are considered US Persons and not subject to export controls. Clearance is a totally different thing that people often confuse with ITAR, which is only granted to US citizens, and though it is possible to grant something somewhat like clearance to non us citizens, it is almost never done.
Edit: Ah I see you are not disagreeing with what I said at all. Apologies, I cannot count the number of times I have seen in reddit and here, where people confuse ITAR with clearance.
"Secret" classification is really mild, about 5 million people have one.
>The law prohibits employers from hiring only U.S. citizens or lawful permanent residents unless required to do so by law, regulation or government contract.
At these workplaces it will be a combination of all 3 of law, regulation, and government contract.