Which is totally within their jurisdiction. The OP admits to being ‘all over the place’ for a few weeks and buying tickets on an ad hoc basis. At the booth/machine, on the train (could get a penalty there) or at the destination (again at risk of a penalty). So your one off is nothing like his...
We ALL know the rules, a ticket is required, end. A one off may be excuseable, but you talk of weeks of boarding trains without a valid ticket purchased in advance. I really have no need to be nasty, but suck it up and pay up, you knew you were doing wrong.