It must be costing them

From their Terms:

DAB Music Player does not host any copyrighted content. Our Service acts as a search and streaming interface that connects to publicly available APIs. We do not store or distribute copyrighted material.

When you open the Webbrowser Developer Tools, Network tab, you can see where it streams from.

When I check on a song, it streams it from a CDN of qobuz (qobuz.com).