Pretty sure the author nailed it, but I hit a paywall before I could copy it.
Anyways:
The only plausible explanation is they licensed this feature, expected huge sales to cover the license, and now can't.
I don't think they have licensed it per use, it was likely a flat fee and now they're desperate to recoup it. They can't do it by sales, so it's monthly fees.
It might not even be this directly, but some other license and this is just where they can squeeze consumers. They might have even just picked what gets used the most