I actually really strongly prefer 0 being identical to unset. If you have an unset state then you have to check if the field is unset every time you use it. Using 0 allows you to make all of your code "just work" when you pass 0 to it so you don't need to check at all.

It's like how in go most structs don't have a constructor, they just use the 0 value.

Also oneof is made that way so that it is backwards compatible to add a new field and make it a oneof with an existing field. Not everything needs to be pure functional programming.