I doubt you legally rode for free. If you ride on the bus or train without a proper proof of purchase, you get a $180 fine.
No, really; there is a shuttle that runs along the strip we were staying at (I think it was a Hyatt Regency?) that actually stops at stops labeled “Free”, and carries you along a specific route (which includes the train station to get to the Mall of America, the Metrodome, and the airport). I’d imagine they all subsidize it - it really was free (even the front of the bus says so). For the train, we bought tickets that nobody asks for, and there is definitely a fine if you can’t produce one when asked. When we got there we paid $27 for a shuttle to our hotel; to get back we paid a total of about $6, and it probaby took less time (and was more comfortable - the shuttle was like those in the Caribbean).