The default event bus receives events from AWS services. Under normal circumstances, for one AWS service to talk to another AWS service, either Roles or Resource based policy comes into play. However, for publishing events to an event bus (which is part of AWS EventBridge, a separate service), an EC2 instance does not seem to need a policy added to a role attached to it (allowing it to publish instance state change events to event bus).
Is the configuration for AWS services to publish events to AWS EventBridge handled automatically by AWS behind the hood without needing Roles or Resource based policies?
EC2 instance does not seem to need a policy added to a role attached to it
Your EC2 instance does not publish its state information to EB. It is done by EC2 service itself in the backend. Thus you do not need to add any instance role for that, because EC2 instance does not take part in this process.
In contrast, if you want your application running on the EC2 to publish events onto EB bus, then instance role will be required.
I can't comment on all possible services in AWS if the behavior is same, but I would think that most operate in the same way as the EC2 service.