Or that, either way, my point is valid. When they replace any part, they don't let you walk away with the defective one.
Technically it depends on what part they replace. If it is a major part, they do send it back to possibly get refurbished by the manufacturer or recycled. Other, smaller parts like cables or similar things, generally get tossed or are not required to be returned in any way, however you still don't get them back (unless maybe from a nice person at the Store).
Apple can and should let you redownload purchased items from their iTunes store. However, due to physical limitations of time and space, you can not re-download anything that they do not offer at the iTunes store anymore. So if that album you downloaded years ago is not being sold at the iTunes store then you can not get it back.
Apple, as with all computer/hard drive manufacturers recommend performing backups. Just look through any of their manuals or at any major system update Apple has released and it will tell you as much. There is nothing else you can do at this point, and as with all major parts replaced under warranty (by Apple, if not most companies replacing similar items), you do not, nor have you ever been able to retrieve it after the fact for any use.
As with others here, you have unfortunately learned this the hard way. Always perform a backup if you want to keep your data.
As an added bonus for rereading that last part of the OP's comment:
Their "protection" plan prevents you from being charged for anything that goes wrong with "their" machines during the warranty period. Any subsequent data you place on it is "yours" and therefor "your" responsibility and should be regarded as such.
If you miss work because your car gets a flat tire, the car manufacturer can not reimburse you for missed time. Or better yet, if your building gets burnt down, no one can help you get your files back unless you had an offsite backup. Even if it was a negligent worker who shorted a wire and caused it.