Even if it wasn't a licensing issue, I can kind of understand why they wouldn't sell the AC5 port separately, at least not right away.
1. You keep the sanctity of the preorder bonus. Bandai/Namco are one of the few companies that really don't release preorder bonuses separately.
2. This entices fans to buy right away instead of awaiting the inevitable Bandai/Namco pricedrop.
3. Say you loved AC5, yet somehow you missed this window. If AC5 were available for cheaper, maybe you get tempted to buy that instead of a full priced, newer AC game. Let that tide you over until a price drop on the newer game.
Only reason I came to this conclusion is the fact that I played AC5 for like five minutes before I skipped over to AC7. I'm not playing both games at the same time, and with the narrative not being super connected, I can't really justify playing the port first. I'm probably missing out but I'll go back to it later.
No excuse for how this is being handled, though. Still not as bad as that crappy Ni No Kuni Wizard edition fiasco from their own website. I still have that guide they gave us because they screwed up. Still shrinkwrapped. Because I had already bought the guide.