Question is in the title… It really should just… not do that.
Unfortunately it seems there’s no way to tell macOS to not make the new installed app the default app for the file types it can read. When the app bundle is created, SpectraLayers just declare the file types it can read, so that the users can directly open them in SpectraLayers with a right click in the Finder.
However you remain free to set whatever app you want as default.
It’s not a right click in Finder.
Opening any FLAC file directly (i.e. double left clicking a file) opens it up in SpectraLayers [by default] instead of the media player. I’m fine with it registering itself as a compatible application, but not stealing the file association for consumer media files that I often play from the finder. Otherwise, Steinberg should remove those associations and just allow people to open these files from within the application itself.
It’s no different than all MP3 files opening in Cubase simply because you installed the application.
This didn’t happen after installing Cubase or WaveLab, so clearly it is not a normal side-product of installing an application [bundle].