Wings of Redemption
All characters capable of making unarmed attacks are considered to be proficient with them. This includes Druids, Monks, and Wizards which are not given proficiency with all simple weapons but rather cherry-pick and fail to specify Unarmed Attacks.
This is a simple extension of the rule that all creatures are proficient with the natural attacks of their natural form.
Note: Not all creatures are capable of making unarmed attacks. In general, creatures with natural attacks do not have unarmed attacks. However, see Natural Attacks for a rough rule of thumb of what to do when spells like Polymorph come into play.