Why are @key annotations enfored for types implementing interface?

[klys-equinix]

I have an interface with an entity resolver like

interface Asset @key(fields: "id") {
    id: ID! # read only
    ...
}

And several types implementing it. Some of them are referenced from external schemas, and so they get their own @EntityMapping,
but most are not.
If I do not add a @key annotation, i will get

Caused by: com.apollographql.federation.graphqljava.exceptions.MissingKeyException: Object Ups does not specify @key fields specified by its interface Asset

for

type Ups implements Asset & ElecAsset {
    id: ID!

Why is it enforced like that?