it's also illegal but you don't hear much about it because those are analog copies.  One of the properties of analog media is that copying degrades the quality so by the time you're making a copy of a copy of a copy, etc it sounds pretty crappy.  MP3s on the other hand is digital.  every copy is exactly like the original.   This is why people aren't flocking to buy someone's vcr copied tapes (also because the internet is an incredibly efficient method of distribution whereby shipping vcr tapes which aren't great quality and will degrade over time isn't generally worth the effort or time)