You can give a countless number of excuses. Also I don’t think they can just stop dozens of people getting off a bus. Why would anyone stop and have their time wasted once they are off a bus? Besides unless they have inspectors on board to check every individual who boards the bus, many people can get away without paying.
Even if inspectors were on throughout the route things still get messy. The busses are very long and people board from all doors. There is no way to keep track of everyone especially with constant boarding and unboarding. And with random checks they are for sure missing people getting on for only a few stops. The whole system is flawed and a real money gap. Strange how they came up with this plan.