It was discussed quite a bit at the time when the feature was added & the default:on was decided as the better option so that monitors with speakers / hdmi pass through will keep working & allow audio to continue.
I get it's a bad idea from a laptop usage (which is tricky to determine other than by looking for an active battery & even then that might not be right) though d I was getting more complaints about playback just stopping that meant keeping the device in an active mode is the lesser of two evils (plus it's a simple check box

).
There's apparently a way on more recent Windows versions if the program is capable of doing it to run in a low power mode but can keep playback going though I don't believe that's something I can easily support even once I've dropped the likes of the Winamp core due to it being a UWP solution & I'm not sure I want to go down that route tbh.
-dro