People make mistakes.
Accidents happen. Even
brain surgeons make mistakes. Even
noders make mistakes. If you give out millions of cups of
coffee every day, thousands of people - even reasonably careful ones - are going to end up spilling it. Any product (like coffee at 85 degrees
Centigrade) that can inflict
major bodily harm as a result of a common and easily-made mistake is criminally dangerous; most such products have been
banned.
To go into this in a little more detail: people will immediately bring up counterexamples of chainsaws, knives, drills and such like. Clearly, for each type of product, there is a level of caution and training that is generally agreed to be needed for anyone to use it safely. And these devices have safety features thrown in, for example the sticking-out bit of plastic or metal that stops your fingers slipping onto the blade of a carving knife. By contrast, a hammer isn't going to cause major tissue damage if accidentally misapplied, so hammers have no major safety features and people swing them around with abandon. (Or at least, the level of tissue damage is predictable and strictly proportional to the force applied by the user.)
Also, the level of carefulness that can be expected from the user is different in different situations: you would be crazy to (for example) serve takeaway half chickens with complimentary carving knife from a drive-in restaurant, since although a carving knife is perfectly safe when used on a sturdy, stationary table, it is a hazard when passed through the window of a vehicle.
So, to return to the case under discussion. What is the level of care and attention that one can reasonably expect from someone at a drive-through? Pretty low. Customers (at least those not in trucks/SUVs) are in an awkward position without much freedom of movement, trying to take their food and drink from the side and above through a relatively small window. Inside the car, there are precious few flat, solid surfaces to put things on. This makes spillages much more likely (hence the large number of similar cases of scalding due to McDonald's coffee) and makes it imperative that anything spillable not be dangerous.
And what is the level of caution and training necessary to "use" a cup of coffee? At the usual temperature (60 degrees Centigrade), again the answer is, pretty low. Suppose you ask, if the woman had been in the habit of putting coffee between her legs, why had such an accident not happened before? The answer is, even if it had happened before, it would not have mattered - if the temperature had been that recommended by the manufacturer of the coffee machine, rather than 25 degrees (45 degrees Fahrenheit) hotter! It would have been a nasty shock but no skin grafts would been required. In doing what she did, the woman almost certainly allowed for a small risk of spilling the coffee, and balanced it against the benefits of adding sugar and cream. This was a reasonable decision, on the reasonable assumption, based on previous experience that the coffee would not be hot enough to cause serious burns. You don't get to age 79 without being able to assess risk. In fact, the same accident could happen to anyone who has wrestled with the lid of a coffee cup trying to add sugar and cream.
Particularly important in the trial was the evidence of McDonald's design engineers, who developed the lid in question. They had to admit that they couldn't operate the lid properly themselves without risking spillage. Clearly, the lid is intended as a safety feature: it should prevent spillage when on, making it more acceptable to serve coffee at high temperatures, and should be removable without excessive effort. However, McDonald's own designers knew that it didn't live up to this specification. One might make an analogy with a saw blade: suppose a company sold (without instructions) a saw with a blade that either fell out unexpectedly, or was so awkward to remove that many users ended up cutting themselves.
Then we come to the little signs "Warning. This product may be hot" or suchlike. Do they alter the morality of the situation? Not much, since a) everyone ignores them or treats them as a joke (even some McDonald's employees), and b) there is "hot" and there is so-hot-that-it-readily-causes-third-degree-burns, and it's not clear which is meant.
And the moral of the story is - Products are made for real human beings, not for idealised perfect beings who never make mistakes; people are not made for products.
The intention of this writeup is to put the case into the context of what happens in other situations where products are potentially dangerous, to make it clear what principles are involved in deciding who has the moral advantage, and to show what is a reasonable analogy and what is not.