How to calculate the Probability for the Unconditional Node in the Bayesian Belief Network?
As the comment says, Burglary and Earthquake only have prior probabilities in the Bayes net. These priors are not calculated from other variables/distributions. They are just assumptions in this example, or, sugarcoated, expert knowledge. In fact, it is often hard to find good priors (from expert knowledge). Extracting them from large non-biased datasets can help. Anyway, one needs to understand the dataset and the context/circumstances/constraints of its collection.