That's fair so long as it was made clear at the time of purchase that you aren't buying the song, and that you're only renting it for a while. If they didn't put a clear explanation that you might lose access on the page when you clicked buy, and not buried deep in a 50 page EULA, then it is entirely reasonable to assume that Apple can afford to keep a 5MB file available for streaming forever, or until Apple closes down its streaming services.
Not necessarily. What if the store didn't have to do that?