Mac OS X places installed fonts in the user's Library folder to differentiate between system fonts (located in
/System/Library/Fonts), OS fonts (
/Library/Fonts) and user installed fonts (
You can change this by open Font Book preferences (File>Preferences) and changing the "Default Install Location" option from User to Computer.
Hope this helps.
Added on 1/12/12 at 2:35pm:
Forgot to mention - If you're not able to delete the font from Font Book, try removing it from the
/Users/~/Library/Fonts directory and emptying the trash. Also, make sure it's not a font that is also in the
/Library/Fonts directory (preinstalled).