> At a business level, can you share why the ToS forbids third party clients at all? We all know that "trusting the client" is not a viable security plan, so why does it matter what client people use?

Because if something breaks for a user and they complain, the company cannot diagnose it or fix it. Simply dealing with the complaints would be an extra cost on the company.

And when they decide to change part of the API, you have an unknown number of users that would be broken.