Being able to vote. Consular services. The American military keeping the peace worldwide. You can quibble over the value of these services, but there are logical reasons why countries may want to tax overseas citizens who retain citizenship.
I once worked with an American guy who served 5 years in the French Foreign Legion. This poor guy played hell when he returned to the US after an almost 5.5 year absence. The Legion gave him a new name during his service and French citizenship upon completion of his tour, and when he returned to the US, he said they treated him like a pariah. The IRS audited him, yet did not assess him any levies. It was a nightmare for him that took over 6 months to sort out--with help from a tax attorney.