[Kip Currier: Trademark law is the issue with the nicknames; not copyright law.]
"According to Matt Breen of Philly.com, two players, Zach Eflin and Hoby Milner had their nickname requests denied due to trouble with copyrights. Eflin was hoping to be known as “Led Zeflin” and Milner wanted to use the Force as “Hoby Wan Kenobi.”
Neither player was listed on the Phillies.com article displaying the nicknames the players on the team selected."
