It's true only if there is hypocrisy in when we ask for silence and when not to. I hope that I can say truthfully that I have never celebrated anyone's death. There is always something sad when a life ends. If it were that of a good person, we can grieve that he will no longer be with us. If it were an enemy, that he will never face justice, and if just an innocent, then somewhere, someone will be upset at their passing.
Once the initial grief is over people can destroy what that person stood for all they like. In some cases, e.g. Jimmy Savile, it's clearly necessary to examine closely a person's legacy. Actually I'm sad, too, at Savile's death. With his death died the hopes of true justice for all his victims.
So the article is right if people are inconsistent in applying the rule of etiquette. But I think it's wrong in general. No death was ever worth celebrating, if nothing else because it came too late to make a difference anyway. What damage Thatcher did was over long before yesterday, and it's sad that people are still better enough 23 years later to feel joyous at the passing of an old woman lonely in a hotel bed.