Imgur serves advertisements when you go to their page. But you don't get them through the API.
https://api.imgur.com/#commercial
> Your application is commercial if you're making any money with it (which includes in-app advertising), if you plan on making any money with it, or if it belongs to a commercial organization.
Calling an API and monetizing the front end it is fine. The money that Imgur loses from ads is made up in the API call monetization.
The Reddit APIs don't prohibit serving advertisements or any other form of monetization of the use of the APIs and mention it as something that your app would need to do.
https://www.redditinc.com/policies/data-api-terms
You will disclose in your App through a privacy policy how you collect, use, store, and disclose data collected from your App Users and other visitors, including, where applicable, that third parties (e.g., advertisers) may serve content or advertisements and collect information directly from your App Users and other visitors that may include the use of cookies. In addition, by using the Data APIs, Reddit may use submitted information in accordance with our Privacy Policy.
It was about maximizing the value of the IPO by ensuring investors would be minimally aware of the failures of the app and new Reddit, by ensuring as few 3rd party products exist as comparatives to the first party offerings.
It could be argued to be part of a conspiracy to defraud investors by ensuring they were incapable of being fully informed....