There is a downside of making buses free, similar to the experience of cities which stopped enforcing "turnstile hopping" for trains, which is that it attracts a small number of hostile and malicious riders. An advantage of treating transit as a public good means this downside becomes an empirical question, not a moral one: Which approach leads to more ridership? In some cases, enforcing fares leads to more ridership by increasing safety and decreasing the amount of time spent cleaning up befouled surfaces.

Sorry, I don't believe you that what would stop those "hostile and malicious riders" is the $2 fare to hop on.

Let's use Seattle as an example. We tap orca cards to pay to get on and recently debit cards. This doesn't in fact keep the crazy people from getting on without paying at all. Only cops/security actually prevent this and most of the time we do a whole lot of nothing.

We could offer free ridership but still use orca cards and ban people who misbehave or befoul the place. Whether we keep problem children off appears to be wholly orthogonal.