The threshold isn't quite 50%+. If one of the 49% nodes mines a block with a would-be censored transaction, the censoring notes have to make an economic decision to try to mine a replacement block and then a second block on top of that to rewrite the chain. At 100% this is easy, but at just 51% it's a costly gamble that will frequently fail at a huge dollar cost.
A block costs $200k at current prices. At just over 50% of the network, you are gambling at least two blocks to force a rewrite.