Why are some arrangements available for free, while others have to be bought and paid for?
Contemporary compositions (movie soundtracks, New Age, Rock, Pop, Jazz,...) are protected by law. You can only lawfully own a copy of them if you pay for it. This usually gives you the added benefit of having excellent sheet music quality, optimized for digital use (no hard-to-read scans of yellowing old book pages!). And a fair remuneration for contemporary composers ensures they can keep writing music professionally. Once a composer has been deceased for more than 70 years, his or her music enters the public domain and can be used for free. However, publishing houses can still hold legal rights to certain editions.