It sounds like a bug to me - no promises it'll be fixed, there might be reasons I don't know of - but I will file a bug and we will look into this.
Seem this meta field was treated as an "internal" field and not be accessible from Service API until now. However since the concept of market exclusion is well-known and you are asking for this. So we just done a fix (in our development side for now) to include it in GET api. So it could be available for you in next release of service api.
I am trying to update my product availability using the _ExcludedCatalogEntryMarkets MetaField in Service API. I think I can PUT a new field for the product but i can't GET what is already set. There simply is no field in the MetaField list.