This is an interesting question, and worth a much more detailed reply. The difference lies in the evolution of online culture compared to offline culture. The Internet has historically been a medium for the open distribution of information. Most Internet users have been brought up in such a community, and thus most Internet users show a distinct leftist bias (at least as far as IP is concerned) in their views.
The general view has been that once you buy a CD, it is yours to do whatever you want with it. If you want to copy it and distribute it for free, then that's your choice. In reality, you don't buy a copy of the song, only a license to use it for personal use, and under the license, copying and distributing is illegal.
There has been a lot of controversy regarding these licenses. After all, you never sign one when buying a music CD, and even licenses for software products have been mired in all sorts of controversy regarding their legality. It surprises a lot of people when they realise that they don't actually "own" the music CD or software that they bought. Software EULAs in particular add all sorts of absurd conditions in legal jargon that no one has a hope of understanding. Many are surprised to hear that common license agreements prohibit resale of the software, even if you paid for an original copy. Music CD's are even worse, with most people never even seeing the license which is printed in fine print in some corner of the CD or cover.
Intellectual property evolved in the 18th century out of a need to give creators limited monopoly over their creations in order to spur creativity. The reasoning is that if creators have no incentive to create, society as a whole would be poorer for it. The concept is to promote creativity in society, and protecting the rights of the creator is the means to do this, not the goal.
Today, IP is being used (abused?) to squash other creators, and is therefore not serving its original purpose - indeed hindering the purpose - of promoting creativity in society. This is the root cause, the reason why many people are against current IP laws. They feel that they are unfair, and no longer serve their original purpose. They are against organisations like the MPAA and RIAA that seek to use IP solely to enhance their bottom line. They feel that the laws are too restrictive and must be modified or an alternative found (Some are promoting creative commons, which is much less restrictive than all rights reserved, but still protects the creator, as the alternative).
Prominent online blogs and personalities are encouraging people to copy music and movies. They promote the view that if you bought it, then it ought to be yours and you should be allowed to do what you want with it. The unfortunate thing is that many users nowdays don't really realise all this. They just see their friends do it and think that it must be okay. Oh well.
Some references -
- Larry Lessig is a prominent IP professor and lawyer at Stanford Law School, and he is the creator of the Creative Commons. He has written numerous books on these topics.
- The November 2004 copy of the prominent WIRED magazine is titled "Fight for your right to copy". A free online copy is available here.
- The Slashdot - Your rights online section is extremely widely read.
- The Electronic Freedom Foundation is a huge, well funded non-profit for the protection of digital rights.