Three things:
1. It may not be as unchanging as you think. The different translations of the Bible (NIV, ESV, NKJV, NASB, etc.) are a pretty important facet to many, with newer translations coming to prominence in English speaking communities every ten years or so (for better or for worse, but that's another topic). I can't speak to other religious texts, but keeping new translation available could be enough of a reason alone to make an API.
2. Having an API can facilitate internationalization if you wanted to ship a project in other languages.
3. Having an easy way for a computer to look up a particular passage is a non-trivial problem to solve, and something like this could give a huge jump start for hobby/personal projects.
Yeah you could just download the Quran and reference it locally or someone could (relatively) easily roll their own version of this, but I think something like this is nice and I can appreciate it's value, even if Islam isn't my cup of tea.
[edited for clarity]