DMCA says: if you want to have a safe harbor, these are the basic things you have to do. You have to name an agent, take notices, takedown promptly, have a repeat infringer policy, etc. If you don't do those things, you lose your safe harbor.
SoundCloud does all those things, so they get the safe harbor.
The additional systems they have chosen to put in place are just that, additional. Adhering to DMCA is not required (though you don't get the safe harbor if you don't) and it's not a ceiling on what kinds of arrangements companies can make with each other.
But again, you're right that it's the thing that burned me here, and I think they merit a lot more scrutiny than they get, and I'm trying to lead that scrutiny more in other places. Prohibiting additional measures would also tangle up other site "guidelines" like the Vimeo one I mentioned, or even YouTube's ability to do content matching and revenue splitting.
You've identified a really key problem though, that those additional rules, whether offensive or inoffensive, are made completely at the discretion of online services and generally without input from users.