He explained that, "for future reference" it was "expected" of programmers to keep real-world samples of their code from proprietary jobs, and that he had stuff he had worked on for a number of large companies.

I'll bet his company lawyer wasn't in the room when he said that, or he'd have gotten taken out and spanked.

I spent many years as a hiring manager. If someone brought proprietary code into an interview, they'd get flunked on the spot. And it did happen a few times. You don't start a trust-based relationship by demonstrating how easily you'll break legal agreements.

At my current job, we ask candidates for a code sample for a generic problem we pose. That keeps things safe and neutral, and helps prevent misrepresentations over authorship. (And we follow-up with some problem solving at a whiteboard for candidates who make it through written screening.)

Edited to add: A lot of people never get training on the legal and ethical aspects of recruiting and hiring, which is a shame, since some people have odd ideas about what's appropriate, and in some places the lines are drawn seemingly arbitrarily (at least in the U.S.). If you find yourself interviewing candidates and you're not sure what's legal to ask, track down someone in your HR department and ask them. If they don't know, work your way up the department until you find someone who does know. Getting a synopsis of the rules shouldn't take more than an hour (in the U.S., at least).