Yeah, but I may be wrong, but isn't it quite hard for Reddit clients to handle this? They'd have to project how many API requests their users are going to make and if they underestimate it's potentially costing them millions. Also, it's difficult to modify app subscription prices constantly, especially for existing subscribers? They could offer in-app purchases of API requests, but that results in quit a bad UX.
If Reddit should charge for API access, why not make it part of Reddit Premium? Reddit gets more premium subscribers and clients don't have to deal with all the complexities of how to handle API request prices.
It reeks of just wanting to destroy third-party clients in favor of their own (terrible) client.