I am baffled by Nature saying would this have been communicated during peer-review this would have been taken into consideration, but because this was discovere

I am baffled by Nature saying would this have been communicated during peer-review this would have been taken into consideration, but because this was discovered after it doesn't count anymore. It's like a teacher saying if you get caught cheating during the exam you will be punished, but if it's after you can get "scott-free."