Good question. I like this discussion.
In cases of existing cultural relations, I'd say there's sufficient record of Betazoid presence in the UFP, that anyone negotiating with the Federation ought be well prepared for the likelihood that they or some other type of telepath in their ranks might potentially be involved. Telepaths are commonplace among them, & we all know it. Buyer beware.
Now as for new relations or 1st contacts, then yes, I'd agree it's somewhat shadier to go into negotiations or meetings without disclosure, but how often are they really doing that? In Troi's case, the only strategic spying she manages is to be a mediocre lie detector, in a person by person situation, & if she's detecting someone being untruthful in their dealings, then is it at all unethical to conceal that ability? It's no longer good faith relations.
I also seem to recall in some good faith exchanges, she did willing disclose her abilities, though examples escape me at the moment. Point being, they're usually pretty mindful & above board with it, which is commendable, given how commonplace it is for them. I'd think it would be much more easy to get desensitized to it, & just forget, but they do tend to bring it up a lot, despite it being quite ordinary for them.
Now... ask me how I feel about Troi's tendency to make herself at home in her
shipmates' emotional innerworkings & I might give you a rather different answer, whether she considers it her job or not. Being maybe the only one we see on board, she does display a rather entitled posture now & then, as if she has a right or freedom to be all up in their consciousnesses, & while much of that might be beyond her control, how she responds or reacts is very much something she should keep a lid on, unless there is legitimate concern for psychiatric issues.
I was very down on how she gave O'Brien a shame glance in
The Wounded, when he had surface some errant hard feelings about his former enemy combatants coming aboard. He did nothing to deserve that, & if that's an example of how she expects she can treat such things, I don't approve.
But again, these are the exceptions, so I don't really harp on her too much about it. It's so damn fictional anyways lol