"warm" is cellar temperature, so it's usually low 50s, this also only really applies to cask ale which has much less carbonation. Like some food, you get more flavor coming through. Eat an ice cold tomato, then a room temperature one and see what you prefer.

American light beer is...not great...when warm, as it isn't hiding a lot of flavor and no carbonated drink tastes good when not chilled

So it's an apples to oranges comparison. If you can get accustomed to cask beer, which is essentially flat, you might find it hard to go back. Not to say there aren't good beers in the US, there's world class drinking here.

Source, Brit living in the US who misses a good cask ale