One thing that should be made clear is that the decision to hold out had not entirely been the band's and it wasn't over philosophical grounds either. It was essentially a money dispute.
There was a legal dispute between Led Zeppelin and Atlantic Records that began in 2002 in which they claimed Atlantic was shafting them on royalties. Furthermore, there was a bidding war between publishers that lasted several years which ended with Warner/Chappell guaranteeing higher royalties than in their prior agreement, and distributing via Warner Music Group.
Normally bands don't have this kind of leverage, because they aren't typically the copyright owners. But when Led Zeppelin originally licensed the songs under Superhype Music, they were in a 26 year agreement at the end of the term of which the copyrights reverted to the members of the band. From there they've provided publishers like Warner Chappell mechanical license to reproduce the works while retaining ownership. But that isn't typically how it works. It could have been because Led Zeppelin's earliest recordings were largely self financed... but when an artist gets an advance from a record label, that is a loan and the record company holds all copyrights in the material even after the loan is repaid through the recoupment process.
Today, few artists are smart enough to demand such agreements and consequently end up indebted to the record labels when sales start to slip.
