USA should remain neutral over a nuclear Iran

Why are we telling Iran that nuclear weapons in their hands would be unacceptable? They are a sovereign nation, they can do what they want. What becomes an issue is when they start talking about using one against another sovereign nation, and that is what we should be addressing, but to simply patronize them and say that their possession of a nuke would be unacceptable seems like a immature way to deal with the issue. When they view us, justifiably, as arrogant, condescending and meddling they are far less likely to respond in a constructive way. If I were Sec of State I would tell them that we, the USA, respect Iran's right as a sovereign nation to pursue whatever policy they deem to be in their self-interest, but that we remain concerned, however, for our allies in the region and that we wish to discuss this issue.