Why no free next gen game/refund ?
Probably because it would work like this...
A few hundred thousand people bought the game on PS3/Xbox360....and many just for the single player....enjoyed it and stuck it in a cupboard to never be heard from again and are currently perfectly happy customers.
A few hundred people played or continued to play the multi player content and therefore would be due refund/free copy of game on next gen, as has been suggested in other posts.
I actually don't see why offering upgrades doesn't make financial sense, if we're talking downloads for those who transfer accounts. A downloaded game, in and of itself, costs basically nothing. At most a couple cents for bandwidth and a tiny bit of labor to get it up on PSN/XBox One Game Store. The charge is to recoup the cost of game design and development, a charge which these players have already paid.
Going by the weekend challenge goals, fully 25% of active console multiplayer is on previous gen consoles.
A bunch of people got to sign up for free multiplayer recently. It was offered because it makes financial sense to get people on multiplayer however they can: a bunch of those people will buy platinum, and the same logic applies to letting current players upgrade. 25% of console players are not going to be spending much platinum now that it's been made clear there's nothing new in the game for them to want out of those chests. Free MP download on next gen for those who transfer their account doesn't cost BW a penny but both makes their customers happier and lets BW make more money off microtransactions.
As far as the possibility you posit that a bunch of single player people who stuck it in the cupboard would run out and sign up for their free next gen copy:
1. If they stuck it in the cupboard and don't care about it anymore, many probably wouldn't come across the offer.
2. If they do come across the offer and get a free download of the game... so? Isn't that a good thing? They stuck it in the cupboard; they wouldn't have gone out and bought another copy of the game anyways, BW wouldn't be losing any opportunity cost on that transaction. Now that these players have downloaded a new copy of the game, though, some of them will want to play some more, which means some of them will buy DLC and add-on packs that they otherwise wouldn't have. The download cost BW nothing since these people already bought the game before, and now they're actually making money on it.
3. If that's still not good enough and they're freaked that people will not want to play more of their game, fine, tie a new copy of the game to downloading the as-yet-unreleased DLC. BW makes money they wouldn't have otherwise, last gen customers get to continue their relationship with the game and the company.
I'm not in the industry, and I don't rule out that there's some missing step that I don't know about that suddenly makes this horribly unprofitable. But from here, it looks to me like offering upgrades for existing customers is at the very least financially neutral for BW, but more likely this would be net positive.