Unredeemed miles are a liability, but that's an accounting term. It doesn't have a negative connotation in this context. UA wants to sell as many miles as they can, at as high a price as they can -and- they want to maximize the bounty that Chase pays them for each new co-branded account. Mileage Plus also wants to maximize customer engagement, and they might conclude that a multi-partner transfer program doesn't achieve that objective.