Because... parades. Eminently UN-loopy analogy.
That is why we have a parade. We who are not always honorable show our appreciation for honor. We who are not always holy show our reverence for holiness. We who are small pay our respects to what is great. We who have received great benefits show some modest gratitude for those who have conferred them upon us....La Rochefoucauld says that hypocrisy is the homage vice pays to virtue.
Now suppose that the parade were ostensibly held to celebrate the feast day of a saint, and that a leader of the saint’s faith were to occupy the seat of honor. That would not be a case of sin carrying sanctity. It would be a case of sin marching right over the backside and the head of sanctity.
That's unfair, it's wrong -- the homage vice pays is acknowledging virtue for what it is, even if Vice can't live up to Virtue's standards.
Because we all have vices, the fact is, we all sin.
But no, we celebrate the fact.