A big chunk of the national media establishment do live and work in New York. As such, they understand that the subway is massive, it's how people get to and from their jobs, that most people are going to pay little attention to these warnings (or even be aware of them). The hysterical reporting is ridiculous.
How my roommate and I found out about the subway threat: We called our moms. They live in Chicago and Dallas. When we did get on the subway I saw all of two cops which is pretty standard and NOBODY was conducting searches. In fact it was the fastest commute we've had all summer. How seriously can we take things if there is no real attempt to protect us?