That would only work if the average user with Search + API used the same "amount" (Costs to Kagi) as the average account using Search only. If it nets out to a higher cost, Kagi is making less out of the same subscription amount.

Especially if the API induces demand.

(Don't shoot the messenger, just trying to explain why it makes sense that they're not doing this)

I think the only way to make it not harmful to their finances would be if they lowered the cap (even for human use) to where heavy users and API users alike ended up paying 'overages' constantly. Everybody who has studied consumer behavior generally finds that people hate overage charges to an irrational extent[1] so that would be a terrible move compared to just saying "API users, you've got to pay extra, sorry."

[1] Meaning, they are more satisfied with an 'unlimited' or just way-more-than-you'll-ever-need option, even when it costs them more when averaged over even a few months. I'm not saying this makes you an irrational person -- I have the same illogical feelings myself.

That's not really illogical, it's just because many companies use the fine print to fleece customers that are slightly above the limit. Think Ryanair. Or mobile roaming.

This is why unlimited or at least a service stop instead of overage is highly preferable. It's the reason I only use prepaid mobile plans too so they can never overcharge me (and it's cheaper than a contract too)