It's worth reading this article from The Transport Politic, which looks at the ways some transit systems seek to promote social equity through transit discounts. While all five of the transit systems they look at offer discounts for the elderly, children, grade school students, and the disabled (as does King County Metro), those in London and Paris, unlike the three US systems included in the survey, also offer discounts based on economic status. Both cities offer free or discounted fares to people in poverty, the unemployed, and university students; in addition, London offers free fare to veterans, and Paris offers discounted fare to adults with large (three children or more) families.

The article concludes:

It would be difficult to argue that transportation should be reserved for only those who can afford it, and therefore fare schemes that incorporate the needs of the poorest are necessary. Not only should we be pushing vigorously for more transit, but we should be asking for cheaper transit, at least for those without good-paying jobs.

... To those who argue that able-bodied adults have a responsibility to find the funds to pay for their transportation, I suggest that our country doesn’t provide as many opportunities as we often claim it does. Even those who work minimum-wage jobs — consenting to our government’s rabid mission to get people off welfare — spend too much of their limited incomes paying to get around.

In this time of mass unemployment and reduced incomes all around, we must work to reduce fares for people who cannot always afford the mobility options transit offers.

A society is only as strong as are its least fortunate.

I'll go one further and say that all transit should be free and paid for through progressive taxes (i.e. not the sales tax) outside the farebox. The transit system is a social good. More people would use it if it were a) sufficiently funded (farebox revenues make up a paltry percentage of any transit system's budget; a stable funding source would ensure better, more reliable service); and b) convenient and affordable enough that taking transit, not hopping in a car, became the default decision.