Howdy Adam,
Welcome to our support forums and thanks for reaching out to us.
I think there’s a confusion on this, The Events Calendar does not add any user roles, searching a bit it appears to be that another calendar plugin does it.
Our plugin relies on WordPress user registration, so you can change that in ‘WP-Admin > Settings > General > New User Default Role’. Not sure if you will encounter any problems with ‘Event Contributor’ role, but you might want to run a quick test on this.
Please let me know about this,
Best,
Nico