On new events this would default to the user and 'posted by' would not be editable.

Once an event is edited it would default to the editing user and not be editable. 

It is important to identify who sets given dates because of various implications that can occur.  In order to collaborate with that person we need to know who posted the event.

Oftentimes the contact for the event is not the same as the person originally putting in a date.

Currently you cannot identify who posted a calendar date until the date appears on Recent Activity.

Thanks!