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In the EU you used to have within-the-EU flights where you wouldn't be asked for an ID once. Scan your ticket and there you go. Maybe you bought this PDF off a weird looking guy in front of the airport, who cares. They now check it more often than not, but it still happens you fly without a single ID check.

So, I'd assume for a national US flight it probably makes sense nobody needed to actually track his ID for immigration purposes, and so a short look without a digital scan is enough. And a tired TSA employee just didn't care at all on that day.