You want to figure out whether to do as the oracle asks or not. To do this, you would like to predict what will happen in each case. But you have no evidence concerning the case where you don't do as it asks, because so far everyone has obliged. So, e.g., Pr(something good happens | decline oracle's request) has Pr(decline oracle's request) in the denominator, and that's zero.
I think you can say something similar about the original problem. P(decline oracle's request) can (for the new problem) also be phrased as P(oracle is wrong). And P(oracle is wrong) is zero in both problems; there's no evidence in either the original problem or the new problem concerning the case where the oracle is wrong.
Of course, the usual Newcomb arguments apply about why you shouldn't consider the case where the oracle is wrong, but they don't distinguish the problems.
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