Usage of anyOf/oneOf/allOf

While following schema is valid in Open-API:

. . .
components:
  schemas:
    MyModel:
      description: One Model to rule them all.
      properties:
        id:
          anyOf:
          - description: Property. Identifier format of any NGSI entity
            maxLength: 256
            minLength: 1
            pattern: ^[\w\-\.\{\}\$\+\*\[\]`|~^@!,:\\]+$
            type: string
          - description: Property. Identifier format of any NGSI entity
            format: uri
            type: string
          description: Unique identifier of the entity
. . .

...the same definition is not valid against Swagger 2.0 (the editor complains about "anyOf" is an additional property and not allowed).

Structural error at definitions.MyModel.properties.id
should NOT have additional properties additionalProperty: anyOf

Why is that difference between Open-API and Swagger - why does Swagger not follow Open-API at that point?