Because this is how outlook works. Every new Verse or Notes user expects the invitation to disappear from their inbox after they accept it. When it doesn't, their next move is to delete it which effectively deletes it from their calendar and leads to missed meetings. Having this simple thing work differently than Outlook causes total chaos. Administrators can set this default with a policy but they all have to learn the hard way why that policy is necessary. If people want to leave them in the inbox they can change the setting, but the default should be at the setting that causes the least harm.