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You could, but with EU and other legislations you'd have to make it clear it's an ad, so in API-speak, there'd be a "is_sponsored=true".

That makes it too easy for API consumers to ignore. (Compared to human eyeballs browsing a webpage)

But you could make it part of the ToS: if you're using the API to present posts to users, you MUST show all the posts in the list, including the is_sponsored=true ones.

That's a good point, UK Government page on "Guidance: Hidden ads: Being clear with your audience":

https://www.gov.uk/government/publications/social-media-endo...

Even though reddit probably don't tag the ads on their website deliberately, it's still really easy to block them in a browser

Even so, how would you track impressions?

Why can’t you?

“Sent from my iPhone.”

Any time I see "Sent from my iPhone" I assume the sender is a fool

Are you missing the point that adverts can be inline with normal content and therefore can be included in an API result?

Yet.