From what I recall fares don't even come close to covering the costs of public transport in most cases, so whether it is 50c or $1 or $5 or free is likely immaterial, and more about using the fare structure as a way to encourage and/or disencourage the behaviour you want or don't want. And also about using it as a simple way to track usage (as other commentators have suggested) while still being essentially free, without overhauling the whole system.