Tea does not need sugar, whether hot or iced (at least I don't know anyone irl that puts sugar in tea (actually I just thought of one person but I don't like them or talk to them much)).
Sweetened drinks are extremely unhealthy and there's probably a reason most places in the US serve unsweetened ice tea.
On hot summer days I sometimes drink 3l of ice tea. That would be extremely bad if it was sweetened.
In my head (as a German), ice tea is almost always store bought like lipton ice tea. You very rarely get anything else anywhere. Restaurants don’t usually serve home made ice tea. It’s a soft drink. When I make myself tea at home, it’s almost always hot. Unsweetened black tea just tastes awful when it’s cold. The hot tea sometimes gets sugar, like when I make myself lemon tea (black tea + lemon juice + sugar), although I do like to use stevia instead of sugar for the same health reasons because I sometimes drink 2-3 pots (1.5l each) a day in winter and that would indeed be a lot of sugar.
When I have ice tea it's usually a fruity black tea, still no sugar though.
If my (hot) black tea gets too strong (I have a tendency to forget about the strainer) I just add (extra) milk, though that doesn't sound like it would work for ice tea.
If it’s the USA, then “iced tea” may actually mean “sweet tea” (an American South tradition), which is often prepared something like this:
bring 1/2 gallon (1.9L) water to a boil
place 8 large black tea bags in a 1 gallon (3.8L) pitcher
pour boiling water over the tea bags in the pitcher
steep 10-15 minutes, then remove tea bags from the pitcher
add 1 dry cup (220g) granulated sugar
stir the slurry until sugar is dissolved
fill the pitcher to the top with ice cubes
wait 20 minutes for ice to chill and dilute the tea, gently stir again
serve
It may be a stronger tea, but so much sugar gets added (probably 3x what would be used to sweeten tea served hot) that you typically don’t notice any bitterness.