It's interesting. With regards to the "ice tea," would you not say "lemon tea," rather than "lemoned tea"?
Personally, I would say "tea with lemon", but that's a really good question. Maybe in that case you could say that lemon is an adjective. "Lemon tea" in the same way as you would say "hot tea". And you could say "sweetened tea" if you really wanted to but "sweet tea" is okay because the tea is now sweet. The adjective "sweet" modifies the noun "tea".
I really could do without the phrase "unsweet tea", though. That's just annoying. It kind of implies that sweet (or sweetened) tea is the default, when really, when you make tea it isn't sweetened and if you want it to be, you have to add something to it.
And McDonald's new "diet sweet tea" really has me confused. How is that different from taking regular tea and adding fake sugar to it?