Apps using reddit's API (e.g. Apollo) don't display reddit's ads. Imagine you run a bar & serve free beer. You can do this with this one simple trick: constantly spamming ads on every TV (display panel) in your bar. Advertisers pay for the beer & your overheads.
Along comes a guy with a garden hose, sticks it in your keg, and starts siphoning your beer to the bar he built across the street. His place is much better than yours, the alcoholics don't have to watch ads to get drunk.
wat do?
web scraping which is objectively more resource intensive
Pretty sure app devs are not not_scraping out of concern for reddit. If they could, they would (I know I would).