Twitter wants to give you access to the data. Their client is their main product. Thus, every 3rd party client is competing with their main product, that seems to be a fact.

Theres no way to stop you from building one anyway, twitter knows that. If you go against their rules, you're a revolutionary, and if you win that revolution, they'll have to deal with you.

You can't expect however, that the incumbent is going to go around encouraging revolutions against themselves.

The only alternative is to encourage everyone to make clients, at which point, they're just a big cloud xmpp server to the world.