The customer is a working musician or music teacher who needs a legible, properly formatted score today for a lesson or performance and is tired of squinting at a barely-readable free PDF. Money is a per-score sale price, priced low enough to beat the hassle of manually re-engraving a bad scan but high enough to reflect the real engraving labor involved.
The differentiator against truly free public domain scores isn't legality, since both are legal to use, it's simply usability a clean, modern, properly formatted score that a working musician can sight-read without frustration is worth paying a small fee for when the free alternative genuinely costs real time and eye strain to work around.