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That's usually the political calculation for free fares (aside from the virtue signaling aspect) - free fares = more riders = more political support for increased public funding.

The potential peril is that compared to the counterfactual of spending this money on improved service, free fares may attract few new riders or fail to generate additional political capital, or even lead to reduced funding if transit starts to be seen as a welfare expenditure rather than a system for everyone that pays for itself to some degree.