@silverpill ActivityStreams allows @context to be missing, too:
When a JSON-LD enabled Activity Streams 2.0 implementation encounters a JSON document identified using the "application/activity+json" MIME media type, and that document does not contain a @context property whose value includes a reference to the normative Activity Streams 2.0 JSON-LD @context definition, the implementation MUST assume that the normative @context definition still applies.