The fraud was happening with being able to buy tickets on the trains... People wouldn't buy a ticket and on the slim chance they would get checked then offered to buy one with some lame excuse why they couldn't before.

At first they only had a small surcharge for buying on the train, but when abuse was rampant they increased this significantly. Only to have a lot of problems when people would object to the (substantial) additional charges. So no more tickets on the trains.....

And now it's a moot point since the access to the stations is going to be to electronically controlled more and more. No ticket, no access to the station and trains. That's why the machines are at every station, it only takes a few minutes to buy a ticket. Other forms of tickets don't really fit this (unmanned) approach, that's probably why they don't promote online tickets for domestic tickets.