"Its pretty simple really, the offender just follows you real closely through the gate."
This is the same method I've seen in Paris. "Often people sneak in for free behind someone who has paid."*
I've been warned that pickpockets in Paris will crowd in behind you and try to dip your back trouser pocket at the same time. It's probably just a matter of time before the pickpockets in Boston figure that one out, if they haven't already.
[Edit: http://www.flickr.com/photos/danslemetro/89871748/ shows a typical Paris metro fare gate.]