All outgoing connections must be allowed, so that each relay can communicate with every other relay.
In many jurisdictions, Tor relay operators are legally protected by the same common carrier regulations that prevent internet service providers from being held liable for third-party content that passes through their network. Exit relays that filter some traffic would likely forfeit those protections.
Tor promotes free network access without interference. Exit relays must not filter the traffic that passes through them to the internet. Exit relays found to be filtering traffic will get the BadExit flag once detected.