I think this is pretty easily distinguished, though. In Milgram, the subject was in a position to prevent the “victim”’s extreme pain and possible death, at no cost to the subject. In Batson, the subject is in a position to prevent the “victim” from receiving shocks by volunteering to receive those shocks themselves. (Yes, the victim does claim to be unusually averse to shocks, but there’s no real reason for the subject to believe that claim.) I see “help someone by hurting yourself in equal measure” as being a very different ethical proposition from “help someone at no cost to yourself”.
I think this is pretty easily distinguished, though. In Milgram, the subject was in a position to prevent the “victim”’s extreme pain and possible death, at no cost to the subject. In Batson, the subject is in a position to prevent the “victim” from receiving shocks by volunteering to receive those shocks themselves. (Yes, the victim does claim to be unusually averse to shocks, but there’s no real reason for the subject to believe that claim.) I see “help someone by hurting yourself in equal measure” as being a very different ethical proposition from “help someone at no cost to yourself”.