This isn't unique to America though. Here in Austria it's impossible to buy mayonnaise without sugar, for instance. Pesto is getting difficult as well. And many many other things that when you make then yourself would never be sweetened because they are not sweet foods. I don't think we're putting it in plain yogurt yet, but if I had to guess that's probably more due to food labeling laws than the manufacturers not wanting to.

Anyway, here in the EU we have strict rules on when/how foods may be labeled as "no added sugar". I think we should take those rules and turn them around and require "with added sugar" labels on basically everything that is not candy or patisserie, where you would clearly expect sugar as a main ingredient.