I think a default does actually make sense: if you don’t provide one, people will start by getting confused, and if they get past that will try to decide which version they should use, and probably go with the KJV as a name that they recognise—but the KJV is simply not a good translation any more, mostly because English has changed and it hasn’t but also by reason of its textual sources. The WEB family¹ is superior in both these regards and not obviously inferior in others, while remaining public domain, so defaulting to it does people a service. (Note that in all this I’m assuming that any default will be English. I think that’s a reasonable assumption because of the shape of both the world and the internet.)
¹ “Familiy” because the WEB also has a British/International spelling edition, not present through this API. If anything I think it would make a better default than the American spelling edition. Matches the spelling used by more of the world, and lines up with the KJV better.
Your post is a perfect example of why I think it's a bad idea to have a default. There is a lot of debate over which translations are the most valuable and very little concensus. A lot of organized denominations still lean heavily on KJV, regardless of its relative accuracy. A lot of them take strong opposition to KJV. Even if we all agreed KJV was bad, what do we use instead? NIV? ASV? WEB? There is no sensible default.
Besides, just as a verse is part of a book, so is a book part of a translation. It makes as much sense to declare which bible to query a verse from as it does to declare which book or verses to query for.
If considering all English translations, you would certainly get a wide variety of opinions about what should be the default for everyone, if default there be; yet I suspect that the substantial majority of informed opinions would support the KJV, NIV and ESV, with all others trailing far behind. As regards user-facing systems, BibleGateway.com has long used the NIV as its default. I certainly wouldn’t; I’m not fond of what the NIV has become. All in all, for an API I would agree with you and recommend no default.
But for a site like this, you’re only considering public domain translations. Given that, I think that the WEB is a reasonable default, specifically because of the paucity of available options. And when you have fewer options, I think a default is more useful—“you don’t have the one I want, so what should I use instead?” will cause paralysis that a default resolves.
On the available public domain English BIbles: you’re generally limited to the KJV, a variety of more obscure translations older than the ASV, the ASV, and the WEB. (Fun fact: the old testament of the RSV and the first edition of the new testament are now public domain for most of the world, including Australia where I live. I grew up with the RSV and still use a second edition RSV as my primary Bible.) Out of these, I don’t think there’s much contest: the KJV is popular for historical reasons but a poor default for the aforediscussed reasons; of the obscure translations, most or all are either unconventional in some significant way, or would be widely acknowledged to be superseded by the ASV, by direct or indirect lineage; that leaves the ASV and the WEB which is mostly just a linguistic update of the ASV. That leaves the WEB as a sensible default public domain translation.
¹ “Familiy” because the WEB also has a British/International spelling edition, not present through this API. If anything I think it would make a better default than the American spelling edition. Matches the spelling used by more of the world, and lines up with the KJV better.