bousozoku said:You're right, but about 40 years ago, Spears's was acceptable as an alternative in the U.S.A.
It's still acceptable. It's a stylistic choice. A judgment call. In some ways it's preferable because with Spears' you don't know if two people named Spear are being referred to or one person named Spears, but with Spears's it is clear. But most people do opt for Spears' because it seems more euphonious.