I'm new to the forums, but I work as a software engineer for a company. It's extremely common for Digital Rights Management to be always on. I've never had the problem on my app, (purchased through the play store), but I can understand your frustration.
A really horrible case of this is Sim City from EA.
Poweramp isn't even close to that. I do disagree with always active DRM and would prefer less frequent checks if they happen. Use Wireshark capture packets maybe of the app startup, but I'm guessing it doesn't check often. I have it installed on an old phone in my car that's always on airplane mode, never connected to the internet lately, and it runs fine.
I think you should mention the app version and where you got it from. Maybe it's specific to non play store purchases.
Again, I'm a noob on the forum, but I want this app to be the best.