Don't worry. It's dortmund's business. This team will be always doing it forever. Winning a trophy was just bonus but generating more and more money from selling developed players are very smart.
Great picture, I want the same for Arsenal. These are two clubs that use a cynical (towards their fans) business strategy, but it works. I'm not sure if this is the optimal strategy, after all, Bayern has almost all financial indicators twice as much + a bunch of titles, but apparently Dortmund is not chasing risky money, but prefers reliable ones.