The cheap option would have been generating bespoke practice material: clean loops, no awkward overlaps, every element in its place. The trouble is that such material doesn't teach mixing, because none of the conflicts that make mixing hard ever show up in it. A synthetic guitar never fights the vocal; a guitar recorded in a real room always does.
So the exercises use songs by real independent artists, with their stems separated: room noise, debatable arrangement choices and elements fighting over the same slice of the spectrum included. It's more uncomfortable material to practise on, and it's exactly the kind that teaches something transferable.
And it's paid for. A catalogue of independent music used as teaching material is, for the artist, one more platform where their work plays: every play inside Fix The Mix generates real royalties, and the running total is public on this same page. It isn't a gesture — it's the precondition for asking someone for the stems of their song at all.
These songs are what play in the exercises that work on complete material, such as Mix Space and Stem Stacker.