Except, that minerA can't build off block B1 because it has transactions in it that it doesn't agree with. minerB just has to keep trying to find blocks to put the censored transactions into, while minerA is fighting against the rest of the miners to outpace them. This only really becomes possible at much higher thresholds, e.g. 70/30. Meanwhile, the community will see what is happening and steps may be taken to either soft-fork or hard-fork the bad actors off the network.