It's not a rule at all. Have you heard of otherwise law-abiding and non-suspicious people getting permanent secondary inspection because they refused to answer questions?

Believe it or not, CBP is smart enough not to waste its time with permanently attempting to interrogate those who don't answer questions.

Yes.

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When it comes to not answering questions beyond that on the declaration form, avoiding permanent referral to secondary is not a matter of "exceptions to prove the rule". Rather, permanent referral to secondary would be the exception rather than the common outcome.

Some "otherwise law-abiding and non-suspicious people" get permanent secondary inspection because they answered questions. That happens sometimes too, perhaps even way more often than for not having a willingness to discuss more than what was listed on the printed declaration form.

Not necessarily avoiding anything, But managing expectation's that's covers all possible scenarios regardless of perceived frequency. There's many ways to get a permanent referral. One of them is not answering questions.

Frequency matters in practical terms. There aren't that many ways to get permanently referred to secondary at US ports of entry, but a recognized US citizen's refusal to answer questions beyond that on a printed declaration form isn't generally a direct route to permanent referral to secondary. Most frequently, it isn't.

I don't know that there is such a thing as a permanent referral to secondary inspection.

I agree. The primary officer at passport or customs control handles admissibility and makes the decision whether to refer to secondary, I do not think the electronic system would ever mandate automatic secondary.

That said, the primary officer has access to a lot of information, including automatically determined risk assessments, which they are not going to simply ignore. An officer can see past noncompliance (if it was noted) and decide you're not their job to deal with. And they have to keep the line moving...

I think there may be alerts dictating that certain people be sent to secondary, but I don't think they are ever permanent or lifelong.