I don't follow what you mean by this. The API endpoints that a company provides ought to be secured properly. In practice they might or might not be but obviously they ought to be.
I don't see what that has to do with third party clients though. A third party client is stuck interacting with whatever API the company provides, however secure or insecure it might be.
Have you done much work with authorization? To put it in another way let's say there was a website that said it authorized with Steam. It asked you to put in your steam username and password. Is this secure?
Now let's say that same website instead redirected you back to Steam (properly) and requested authorization on behalf of you. Is this secure?
Now which bucket does this app fall under?
Unauthorized? Hasn't the user explicitly authorized this layer by installing the app?
"Is this secure?" fully depends on what the attack vectors you're considering are. Breach of the server's database? Make it an app instead of a website and make requests directly. Malicious code in the client itself? Make it open source. Now it's even more secure than the official client.
But regardless of all of this, how is it any of the service provider' s business what I do with my login details? It's my data on my account. If I use it in an insecure fashion, that's my problem. I am free to post my login details on Twitter for everyone to see, so why can't I put them in a database on some russian dude's basement server?