Reply to Recording/Ripping Tracks off Videos

I know its not "technically" legal, but there are just some songs that u find on youtube or see online in general that you cant buy on beatport or itunes. How do these guys on youtube record these songs and post them online??

For example, deadmau5 streamed his live set recently on ustream and someone somehow recorded his new song "Raise your weapons" from that stream and posted it on youtube. How can I do that??