Many cultures have the concept of expiation of inadvertant sin. For example,some Native American religions require adherants who have been accidentally exposed to a dead person, to enter a sweat lodge, in order to eliminate every trace of contamination from the body.
Simalarly, a public fast for the Monsey community will be a shared cathartic experience.
The operative words here are SHARED and COMMUNITY. Orthodox Jews really do believe that we are all responsible for each other (this is why so much charity and good works circulate in our community.) When there is a real tragedy in the worldwide Jewish community(not only Orthodox, but all Jews) an untimely death, murder,etc., the entire community mourns, even if the victim is not personally known. When someone in the community triumphs- a Jew wins the Nobel prize, or a gold Olympic medal- we feel proud by association.
In this situation, although the "sin" was obiously inadvertant, we can't help feeling that to some small degree, we share guilt by association.
Finally, religions arent based on reason, they are based on faith and to some degree, on a social contract.
Many of us derive great comfort from our traditions and rituals and could care less what you think of them. I personally think those of you who attack Orthodox practices are quite provincial in questioning a belief system that is not your own (but harms no one outside that belief system.) I am not so conceited that I believe every person in the world should live as I do, but obviously some of you believe just that. You think you are sophisticated, but your judgemental observations are the antithesis.