[UI discussion] editing a calendar day in the past

I’m making a web app similar to a calendar and am seeking some UI advice. I was wondering why existing calendar apps, like Google’s, allow you to edit or delete an event in the past. Is there a compelling reason to allow users to do so? Maybe they should only be allowed to change up to a week in the past. That way, people who they’ve shared their calendar with, won’t see history (whether good or bad events) being altered.