There is nothing wrong with a musician playing copyrighted music - what is wrong is when they perform that music in public. That is why bards are not permitted to perform such music as a performance in FFXIV is technically a "public" performance. Sheet music is available for copyrighted works and they can be performed in all manner of instances but when they drift into public performance and especially where reward is involved then the copyright holder for public performance will want their cut.

Reminds me of an instance when a local school production of the Wizard of Oz was prevented from going ahead because a West End production was happening and the copyright holder pulled the plug on all other performances.