It's not strange at all. This is from the T&C in the original offer email:
Take six qualifying one-way trips on Acela from January 3, 2017 12:00:00am CST through February 28, 2017 11:59:59pm CST and get two free one-way Acela trips.
Take six qualifying one-way trips on Northeast Regional from January 3, 2017 12:00:00am CST through February 28, 2017 11:59:59pm CST and get two free one-way Northeast Regional trips.