From the operators point of view, they are getting paid the same whether it's just 1 person boarding or it goes full, so not their problem if no checks are done.
Some stations may have rail staff who could check tickets, but in most cases if there is anyone (at a major station) it is likely a co-ordinator that works for someone like Transport UK, so they won't be checking tickets either. Of course they could be advising people to have tickets ready, or implying checks might be carried out, but will most likely be bluffing. In some cases, some staff or drivers may feel it isn't fair for people to get a free ride and unofficially check - but they probably shouldn't. Still, their bus, so if a driver wants to play silly buggers then that's between them, the bus company and the TOC to argue and consider what action is necessary.
Some stations will keep standby buses for when they become overcrowded, and clearly if you're letting people travel for free and everyone knows it is free, you'll potentially be leaving people behind. What if those being left behind have tickets and those onboard went for free?
And what about stations where you can pay contactless. There's now no way to tap in or out!
RRBs are a bit of a minefield when it comes to revenue, and I think most operators will essentially allow people to travel for free and hope that if the buses only form part of a longer journey, people will likely buy tickets in those cases. Most people are probably inherently honest and pay because they know that's what is expected.