If I open it in the middle, how do I adjust the dose? For example, if I only want half the amount of sugar, it seems easy enough when opening the stick on one side. If I open it in the middle instead … do I have to keep one side shut? Do I let out half the sugar on each side? What if I want a third? If I open it in the middle, I always have two sides I have to portion. It seems far easier to portion it “the wrong way”. Would you open a Snickers in the middle as well? I completely understand the argument about the little rest of sugar in the tip, but I don’t think it’s that horrible an experience.
Frankly, I don’t think this applies as a bad design example per Don Norman. At its most basic level, good design comes down to discoverability and understandability; how can I interact with the product, and what’s the intended workflow to get to my desired result.
The discoverability is easy enough: it’s a paper stick. There’s content inside. The tearing of the paper is the intuitive affordance that’s immediately obvious. There might be slightly better and slightly worse places to tear the paper, but the end result is mostly the same: the design works. Similarly, the user almost immediately understands what she has to do to get to the end result. I’d argue being able to tear it anywhere makes this design better: there are a lot of plastic wrappings for candy and such that can only be opened on one specific, pre-cut position. The user has to open it the way intended by the designer — but has to actively look for that way. It only becomes more obvious through the use of additional signifiers. I’d argue the sugar stick is much more elegant in that regard.
A bad design would cause frustration or confusion. Take the famous Norman doors: those are doors that are not obvious to open, or even offer confusing signifiers. You walk towards them, expecting a certain behavior, only to be stopped because they wouldn’t open as expected. That’s bad design. Everybody knows how to get to the sugar contents of a sugar stick quick and easy. The more popular way to open them might involve a little bit of hassle with spare sugar grains on your fingers, but I don’t think that affects the experience significantly.