Seriously, _what_ are they gaining by eliminating access to third-party clients? If they want usage data, they already have all the API calls. If they want more ads, they can change the APIs to inject them.
Reddit wants freedom to arbitrarily change the design of their app and placement of ads, etc. Ads are a huge (primary?) source of revenue for them.
If they are tethered to supporting third party clients, it's harder to make reasonable estimates of how many captive users will see ads or new features.
Reddit could enforce ad presentation in third party clients, but to appease advertisers Reddit has to make guarantees around visibility. It's not enough to check if third parties are calling the correct API, they will actually need to regularly audit all third party clients.
It really isn't worth the time or effort if you can just charge third parties the cost to cover loss of ad views.
Except that's not what Reddit is doing here. They're charging 3rd party clients ~21X what they lose in ad views, pricing them completely out of the market.