As Mark Spencer76 pointed out, anything 'fixed in a tangible medium' (in this case mp3 audio files) is copyright protected, so muthafuckas don't try to be stealin' P's songs 'an shit, k? However, to get the full protection of the law, one must register copyrighted materials with the Library of Congress using Form RA (to copyright the song) and/or Form SA (to copyright the recording itself). This usually gets taken care of when a record is going to be released, so probably Spongebath took care of it for the older songs. Then again, I don't know, it does cost $35 PER SONG...
In all seriousness, I see no problem (unless the band feels otherwise) with swapping files that are a) out of print material,

live material, or c) other unreleased material that the band has no plans to release in the future. I do think that it is a big no-no to swap files of material that is currently for sale by the band. This would include anything that is on 'The Beginning' EP or the 'Thursday' 10' single (Hint, hint).
Relevant: Prince, PT Anderson, Punk, Post-Punk, Purple, Party of Five, Peter Swanson, Peter Gabriel-led Genesis, "Peter Panic", Paul's Boutique, Potential Energy, Every Features MB member but me.