It's using what someone made available on certain conditions (e.g. paying for it), while violating those conditions.
The moral argument against it: you are procuring for your own personal enjoyment a recording of a piece of music which cost money to create, and which others are required to pay for. What you are saying is "I deserve to have you (the artist and everyone else involeved - writers, producers, engineers, etc. etc.) spend your time, money, and talent creating a product for me to enjoy any time I want, and I should owe you nothing for your efforts".
The Pirate Bay is raising money for the creation of a 2 hour documentary about the site and it's founders. Give something. For those of you who want to know how to raise money for music, movies, etc., in an environment of file sharing, this is how to do it. This type of monetizing the entertainment business will remove the soulless people currently preying on my 2 young children. The music they like is synthetic, focus-group-derived waste, created by 65 year old, hairy, gold-chain-wearing shysters named Murray from Brooklyn. http://www.kickstarter.com/projects/tpbafk/tpb-afk-the-pirate-bay-away-from-keyboard