That's reasonable. And I agree it's odd. Why should Twitter care if 20% of their userbase is on third party clients with an experience that's not on-brand, if those clients pay a per-user fee?
One explanation: not all users are created equal. It could be that the 20% who use third party clients are more/most engaged. Lassoing them back into the fold could have some strategic benefit.
Twitter can't enforce a "consistent experience" on third party clients, meaning they can't be sure that promoted tweets, accounts, and other forms of generally unwanted advertising will show up in timelines that aren't rendered with Twitter's own code.
They can revoke access for clients that don't comply with the display guidelines. They probably don't want to be in the business of policing 3rd parties like that, but they have the power if they want to use it.
Yes. Additionally, blocking access past 100k also stops one client from being the-tail-that-wags-the-dog too. Imagine a dispute where Twitter faced blocking the chosen mode of access to a significant percentage of users, or caving to the client owner.
One explanation: not all users are created equal. It could be that the 20% who use third party clients are more/most engaged. Lassoing them back into the fold could have some strategic benefit.