  In this work we show that the set of non-signalling resources of a
locally-tomographic generalised probabilistic theory (GPT), such as quantum and
classical theory, coincides with its set of GPT-common-cause realizable
resources, where the common causes come from an associated GPT. From a causal
perspective, this result provides a reason for, in the study of resource
theories of common-cause processes, taking the non-signalling channels as the
resources of the enveloping theory. This answers a critical open question in
Ref.~\cite{schmid2020postquantum}. An immediate corollary of our result is that
every non-signalling assemblage is realizable in a GPT, answering in the
affirmative the question posed in Ref.~\cite{cavalcanti2022post}.
