Again, you're leaning on a moral intuition about theft of physical property. The problem Best Buy have, in that situation, is that they would have to themselves buy another copy of the DVD in order to then have the stock of it to sell. It's shrinkage, like stealing a TV.
This does not pertain to downloading an MP3.
Like I said: make an ethical argument, not a moral argument. Stop attempting to rely on moral intuitions; there are no good moral intuitions for this, any more than there are good moral intuitions for e.g. the ethics of someone who can "fork off" copies of their mind, forcing those copies to merge back together with them. It's just not something that ever came up in our evolution!
This does not pertain to downloading an MP3.
Like I said: make an ethical argument, not a moral argument. Stop attempting to rely on moral intuitions; there are no good moral intuitions for this, any more than there are good moral intuitions for e.g. the ethics of someone who can "fork off" copies of their mind, forcing those copies to merge back together with them. It's just not something that ever came up in our evolution!