I don't think you intended this, but this is a mix of strawman and slippery slope fallacies. It's not "the exact same logic." To begin with, in the OP's story he himself is the "suspect", and he knows he's innocent. There's no moral dilemma over whether to reply to the questions, only one of practical concerns versus abstract ideals.
I don't think the world was in danger of people taking the OP's argument to the witness situation you describe, and thus I don't need that caveat was necessary.
It's easy to see that being innocent doesn't eliminate any moral dilemma over answering questions. Most people are innocent! By answering questions, they're helping the police stop wasting time with them.
Be thankful that we have the right not to answer questions, and, yes, be thankful that crazy guys like this are out there demonstrating that right. But then remember the social contract and your obligations as a citizen; if the police are asking a reasonable question, you should answer.
Again, the OP is not talking about reasonable questions. Whether one is traveling for business or pleasure is not a reasonable thing for a customs cop to inquire a citizen upon entry into its own country. It is indeed none of their business.
I guess they only want to intimidate you and get you talking for a quick psychological profiling. And the OP responds like I wish everyone with nothing to hide did.
And that doesn't mean entrance lines would take forever. Police adapt their routines to the available time and resources.