I see what you are saying but it is right and wrong in a sense.
If it easy to create role and add the role in web part properties for a user.
But if User A and User B are given calender add event role. User A is able to edit/delete other users events also.
A user should only be able to manage his/her events only not other users stuff.
Also, the events design is not very user friendly. Should display events in different color, right now as an audience, I cannot distinguish anything on the calender.