It's important to point out that "free" and "royalty-free" aren't the same thing. Unsplash images are actually in the public domain, whereas "royalty-free" is a license type where an image can be used multiple times for one payment.
1. https://creativecommons.org/share-your-work/public-domain/cc...
Not all countries have a concept of public domain, and many that do have arbitrary restrictions applied to it. In fact, this is one of the reasons you should use CC0 rather than just saying "public domain" or using something like the Unlicense.