Yeah. It would be simpler to talk about causal graphs if the nodes represented only events instead of arbitrary variables, because independence between events i

Yeah. It would be simpler to talk about causal graphs if the nodes represented only events instead of arbitrary variables, because independence between events is much simpler to determine: X and Y are independent iff P(X) * P(Y) = P(X and Y). For events there also exists a measure of dependence: The so-called odds ratio. It is not influenced by the marginal probabilities, unlike Pearson correlation (called "phi coefficient" for events) or pointwise mutual information. Of course in practice events are usually not a possible simplification.