On the contrary. Why should public transport be subsidised?
Let commuters pay what it costs, and cut the tax instead. I speak as someone who would probably wouldn't be affected much either way - I have an annual Travelcard, so that would mean a big rise in fares, but also pay far too much council tax.
There will be special cases - children, the disabled, etc. - but not those on unemployment benefits. There are two ways to incentivise work - the first is to cut taxes and benefit withdrawal rates for the poorest workers; the second is to make it cut benefit entitlements. Centrally, the coalition is doing both (but not enough, I think).
The point is, why should someone who walks to work pay for part of my Tube journey?
I would still fund capital costs for transport infrastructure from the taxpayer - but running costs should come from the users.