Okay, I'm a US citizen (born and raised), so I should be able to satisfactory answer your question. The laws are sort of half-ass backwards if you ask me.
It's not legal to distribute any copied software, period, end of sentence. Free, bought, or otherwise. Even if you own the software already, it would still be illegal to buy a copy of it. Why you ask? Well, copyright laws in the US say that you can do whatever you want (sure sure, like they really mean it [there was an actual trial over this when Playstation emulation started]) with the product as long as you legally bought it and you can do it with ONLY that specific product. You must only restrict the use of that copied CD to yourself, God forbid anyone else lay their eyes upon it.
Case scenero, I have a Win2k CD and my friend also has a Win2k CD. My CD is damaged and no longer works. It would be illegal for me to make a copy of his CD even though I already own the software. But it would be legal for me to make a backup of my CD (if it worked). It would also be illegal if he installed Win2k on my system with his CD.
However, if someone were to burn a backup for you, I'm pretty sure that's legal, so long as they don't take any material off the CD and keep it.
Alright, now here's where the half-ass backwards comes in.
IF you happen upon some warez or pirated software (which of course NEVER happens), it is legal to keep it for 24 hours (trial period), at which time you must delete it and all copies of it after the 24 hour period is over. After the 24 hours it is illegal to have. Now if it's illegal to distribute and illegal to have copies of, why is it legal to have it for 24 hours? LOL
Hmm, those are the only laws I can think of at the moment, if I happen to remember more (because I always abide by them) I'll post them.
Oh yes, just remembered the penalities. For each offenses= you can get up to a $10,000 fine and 5 years in a federal prision. So technicially, for each MP3 you own, that's what you pay.