Firstly, to enforce that reddit now has to police everyone who uses their api, and engage in the inevitable game of whackamole. Charging (a reasonable price!) for API calls and letting the third party app make it back in ad or subscription revenue is a much more sensible way to work. Everyone benefits, and reddit don't need an army of people hunting down TOS evaders.

Secondly, I know I didn't see any reddit ads when I was using Boost for Reddit, so it's actively happening. I was seeing Boost's banner ad down the bottom of the app when I wasn't on my home network.

Lastly, I'm not convinced that surviving on ad revenue is a viable model after seeing dozens of companies go under, if fail to escape the red, either trying to get by on ad revenue or trying to switch to subscription modles because it wasn't working.