[malkia]

Maybe to make the food still tasty. Go to your local store - check all low-fat, 2%-enters, etc. and then full milk - check the sugar content percentage. That's all... At least here in US, Los Angeles - usually it's the normal yogurt/milk (e.g. hihg-fat) that has lower sugary content (percentage-wise) than the rest. Sometimes it's quite significantly lower.

And I don't know how the whole craze about non-plain yogurt came. I'm bulgarian native, and it's "foreign" to me that yogurt is mixed with any sugar at all.

It's still yummy - you can mix it equal portions with water, shake it well - and you get a very good drink. Or make this cold soup - http://en.wikipedia.org/wiki/Tarator

Or just eat it like this. No need for sugar...

[Crake]

I'm American and it took me until last year to learn that yogurt isn't sweet by default. 95% of the grocery store offerings are the sugary kind, and that's all I was ever exposed to as a kid, so I never noticed it. I spent years thinking I hated yogurt only to eventually discover that the unsweetened kind is really delicious. D: