I was being quick and sloppy in my description. Yes: I 'boosted' the North Shore economy a tiny amount by choosing to ride out there and spend money. That ride only happened because of free fare day, an induced trip.
It's not revenue the T is missing out on because I would not have made that trip at all if it was any other day.
I wasn't the only person doing this either. In fact, I ran into another person with exactly the same idea (take a bike to Newburyport for no particular reason) while boarding the exact same train car as me. We both spent money out there. And we also saw a lot of other people getting on and off the train who were clearly people who would not have been there otherwise. Lots of kids, too. I'd see a family of 5 get on-board at one stop, go a short distance, then get off. And they'd be replaced by another family. It was amusing to me. I think the conductors were slightly less amused, one of them complained to me about all of the newbies.