The US government will look out for your interests if an incident goes down in your country of residence. A rare occurrence for most but still a service that some may end up needing\using.
Somehow the UK does it without subjecting all British citizens worldwide to UK taxes.
I read once that UK repatriation was funded as part of the passport fee, but can't verify it. In any case, if this were the justification for the policy, it might be good to know how much it costs the US vs. what extra revenue the US generate from this taxation, and the number of non-resident/non-passport holding Americans are also covered by this repatriation.