P2P servers are not illegal. P2P servers are used all over the place, other than for just obtaining music. It just depends on how they are used. You can actually legally download music through subscription services, such as Real, ITunes, or the new Napster (I think they are around still.. ) These services have agreements with the artist and/or record companies, and they get reimbursed accordingly. I believe that some or most of these services use P2P servers to distribute music, not sure though.
Here's more information about copyright infringement, if you're interested...
http://en.wikipedia.org/wiki/Copyright_infringement
Hope this helps!