e.g. if you expect the average user to use 50% of the search quota / month through the UI, but 100% through the API, then you make the quota shared between the two but double the effect of usage from the API.
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)
[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.
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)