After being chastised for my inappropriate copy-pasting of "Fitts' Law", I looked it up in the 13th Edition of the Chicago Manual of Style and apparently Addison Wesley got it wrong in The Humane Interface.

6.15 Proper names. The general rule covers proper names as well as common, including most names of any length ending in sibilants. For example: Dickens's novels or the Joneses' reputation.

You were right, Bojhan!