[jefftk]

How does the utilitarian know this?

Any morality needs to take into account our uncertainty about claims other people make.

[sigilis]

The mugger has a lapel pin denoting himself as a deontological agent. Lapel pins in these fantasies cannot be forged, I guess.

[jefftk]

If we're assuming unforgeable moral-method pins I don't think we should expect intuitions generated in this sort of thought experiment to be a good guide to what we should actually think or do.