Yes, it's a pity most people don't understand this. The idea actually is a valid concept in justice, but only within the realms you posted: If person A does so

Yes, it's a pity most people don't understand this. The idea actually is a valid concept in justice, but only within the realms you posted: If person A does something illegal and then tries to justify the behaviour X with "But everyone does this!", then this is not a valid argument, because either X is illegal or not. Whataboutism as invented by the economist, though, is just rhetoric nonsense. The USSR was perfectly right in highlighting the human rights atrocities of the US in response to accusations of the US against them. That doesn't make the accusations go away, but it correctly exposes the bigotry.