For people who can't afford the ticket, there can be special prices. For example in my hometown, people with low-income pay less. Kids from low-income families pay nothing.
Making us buy a ticket to use transport, while the poor uses it for free would be on another level.
Public transport is already funded through taxes. We already paid for it.
In Germany the local public transport is only partially funded by taxes. Tickets / Taxes is roughly 50 / 50.
The Autobahn is mostly funded by taxes.