[Programmatic]

The problem with sugar is, IMO, glycemic index. With glucose being an index of 100, no sugar being an index of 0, and something that takes a moderate amount of time to convert from starches/etc to sugar being 50, things that are higher than 50-60 tend to spike your blood sugar and cause insulin to scavenge the sugar out and store it in fat. Doing that often causes things like insulin resistance and obesity as you get hungry again when your blood sugar drops.

Fruits have a surprisingly low glycemic index because they aren't processed and you have to get through fiber and digest the cells before having access to all of the sugar. I've changed my diet to avoid sugar and notice that I very rarely feel actively hungry/crashy vs. before, and most fruit is just fine.

http://www.glycemicedge.com/glycemic-index-chart/

[soreal]

I checked the table you linked and:

Apple: 38 Banana: 51 Pineapple: 66 Watermelon: 72

vs.

Ice Cream (premium): 37 Sponge Cake: 46 Frosted Flakes: 55 Snicker's Bar: 55

Fruits really are very high GI compared to regular snack foods. They're relatively healthy for other reasons though.

[wpietri]

A perhaps better measure is glycemic load, which takes mass into account. 400 calories of frosted flakes is about 100g. 400 calories of watermelon is 1.3kg. Watermelon's glycemic index is worse, but at reasonable serving sizes, fruits generally end up being way better.

[Programmatic]

You also cherry picked, figuratively speaking. More literally, you didn't pick the cherries at an index of 22. There are a lot of fruits in the 20s to 40s. I tend to stay away from both pineapple and watermelon.