I have same question . . . and this one has been circulating in user forums for at least five years (see https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&v ed=0CC4QFjABahUKEwia6orzw9PHAhXFHT4KHdLE…)
The problem is not only just when someone accepts an invitation, but also whenever Mail receives an email with an iCal invitation attached.
How does one actually get Apple to respond to questions like this??