To be fair, just last night I got a 32-ounce drink served to me at a drive through window. Fortunately I saw the loose lid before taking the drink, and was saved from being drenched in sticky soda, and the need for my car to undergo a complete interior cleaning job.
You might think that my experience can't compare to the coffee case, but it actually can. The same mistake that was made with the top on my cold drink was what caused the woman to be doused with scalding coffee. Although I'm not likely to suffer physical injury from spilled soda pop, the cost of removing it from all over the inside of my car does have a price. And although my loss would be more in the hundreds of dollars for custom cleaning, compared to ~$10,000 in medical bills, it's still a liability that the fast food company is liable for when it happens.
The point is that what you call "common sense" is a knife that cuts both ways. (No pun intended, lunatic59!) In the case of the old lady who got burned and myself who might have been drenched, it was common sense for the fast food employee to put the top on properly. Did you think if it from that perspective?
Here's another example. Is it intuitively obvious why you shouldn't stand on top of a step ladder? If they weren't meant to be used that way, how come the top step is there in the first place? Ladder makers could have constructed their ladders with a cross brace that discouraged using it as a step, but they didn't. Was it really a lack of common sense, or did ladder makers really make their products to be used right up to the top step? I'd say that because they slapped on another sticker instead of removing the steps you're not supposed to use, that ladder buyers still want to use those top steps, and ladder makers are happy to oblige. Not so cut-and-dried this time, eh?