The idea is that taking action X or not doing so in any one particular instance can change which history we're enacting, while normal decision theories reason only over the scope of a single choice-instance, with little regard for potential futures about which we don't have specific information encoded in our causal graph.

It seems to me that the impacts of being virtuous on one's potential future is enough to justify being virtuous, and one does not need to take into account the impacts of being virtuous on alternative presents one might have faced instead. (Basically, instead of trusting that Omega would have given you something in an alternate world, you are trusting that human society is perceptive enough to notice and reward enough of your virtues to justify having them.)