Why is highball sweet in Korea?

Every time I drink a highball in Korea, doesn't that sweet taste particularly come to mind? Recently, I ordered a highball at a bar, and it felt very sweet, perhaps because they used tonic water instead of regular soda water. When I asked the waiter about the reason, he said that in Korea, it's common to use tonic water instead of soda water in highballs. This can make the highball taste sweeter. Has anyone had the same experience in other regions? I'm curious about places like Hongdae or Itaewon. If you've tried highballs in other areas, I would appreciate it if you could share what differences you noticed. I would love to hear your experiences and opinions!