It doesn't make sense because you don't understand how registered addresses and taxes work in the former eastern bloc. It's common for people to move to a large city and officially still be registered as living at home. Sometimes it's due to laziness, sometimes they don't want to get the paperwork to get registered at their new address, sometimes they just like having mom take care of all their paperwork/tickets/etc.
Free public transportation is incentivizing these people to register where they actually live. They aren't paying more in taxes, they've just registered as official residents of the city, so their taxes are counted as being paid in the city.