I don't know why, but my experience is quite different. I've seen two therapists and one psych and its always been about me telling them, not them diagnosing me. We have a discussion, they ask questions to help me clarify my thinking, and I tell them the diagnosis. Having said that, I never saw them with the intent of getting letters.
Most recently I was seeing a psych. I've always presented male, and I've said I'm going to continue doing that as long as I can no matter what. There's never been any indication of that being a problem so far as getting letters would have been concerned, had I actually wanted them. The psych challenged me on occasion, but we either discussed it or I told him he was wrong and why and we moved on. I may be exaggerating a bit when I say challenged, as it's never been confrontational (from him, I did it a lot).
I think I'd be pretty annoyed if I ran into your situation. Your trans right, and sane? Where's the <rude-word-here> letter.