Why do configuration spaces not form an operad?

Question

I have seen in multiple sources that the collection of configuration spaces $\{Conf(n,m)\}_{n \in \mathbb{N}}$ where $$Conf(n,m)=\{(x_1, \ldots ,x_n) \in (\mathbb{R}^m)^n | x_i \neq x_j \text{ if } i \neq j\}$$ do not form an operad, (I think this can be stated where we consider configurations in any manifold $M$ rather than necessarily a space $\mathbb{R}^m$, but I have stuck with euclidean vector spaces for now). Does anyone know of a resource where this is proved or could in fact give a proof that it is indeed impossible to create such an operad?