I view it this way... The actual act of taping was illegal, but the tapes being around now are not illegal, and some venues even allowed taping of some shows if the artist OK'ed it, Grateful Dead were big in that, but other bands too, also, if the concert has no copyright, like Led Zep playing XYZ showcase on 2/14/1971, had no official release from a record company, then, no one technically owns that audio that was sent into the air so someone can share it, it can't be sold because John Smith who snuck a tape recorder in would need a copyright of that material to sell it, but because no one has a copyright on the material, anyone can listen for free through any medium as long as it isn't sold by someone who doesn't own the copyright(which in this case, no one does, so no one can sell it)... so selling copies of bootlegs is still illegal, sharing is not, and distrubting free copies of albums isn't legal because some one owns a copyright to it, where no as no one owns a copyright to the bootleg....
hopefully that paragraph/incoherent rambling made some sense