litvishechosid: Welcome to the New York City Public Transit system, used by an average of 5.7 MILLION riders daily! Here is a broader view, that may clarify things for you:
SBS buses are express buses. They make only “select” stops on an otherwise very long route, and are used primarily by commuters; people who are using the bus daily, twice a day. This system has significantly improved the travel time of commuters on heavily traveled long routes that are not serviced by the subway system.
New York has a comprehensive public transit system, but it is quite costly for the typical lower/middle class commuter. The MTA has systems in place to somewhat reduce that very high cost of using public transit. A very high percentage of commuters buy a weekly or monthly unlimited metrocard, for a significant savings, so individual rides, including those on the SBS routes are already prepaid, just by owning either type of unlimited metrocard.
Additionally, most commuters in NYC, especially those who use buses, have a 2-leg commute. Meaning, they use either a bus and a (subway) train, or 2 buses. Even those who are not using a monthly unlimited metrocard, are very likely making use of a free transfer, which is included with almost all trips using a regular pay-per-ride metrocard. The free transfer only works if your metrocard was used (and paid) within the previous 2 hours on a different route.
I suspect that the actuaries who work for the MTA have concluded that the SBS routes are covered at a very high percentage by the fare that was/will be paid on the previous, or connecting route, or by the use of prepaid unlimited monthly metrocards. The amount of people who literally get a “free ride” if they are on an SBS bus, and did not pay is probably negligible. Regular users of the NYC subway system are also familiar with turnstile jumpers, and riders who wait at emergency exit gates, to enter the system without paying. There is a limited amount of oversight available to prevent or prosecute the scofflaws.
I presume a cost/benefit analysis is done routinely, to determine how much money (employed policemen/enforcement officers) to invest, for curbing petty theft by non-paying riders. I hope you sleep better now!