The DOJ spent the past three years pursuing the broader case of Apple's collusion with the book publishers — the publishers immediately settled but Apple kept appealing. Those appeals were finally exhausted earlier this year ( http://www.nytimes.com/2016/03/08/technology/apple-supreme-c... ) so the DOJ might now take the opportunity to tackle Apple's other anti-competitive tactics.
Spotify shouldn't have to spend millions to push it. It's a criminal matter not a civil case. The DOJ is duty-bound to investigate complaints.
https://gigaom.com/2013/08/23/in-ebook-case-doj-claims-apple...