Listeriosis is rare, but if every McDonalds were serving spoiled dairy products, a one-in-a-million chance results in several deaths.
Yes, it's deadly, this is exactly the reason why we should make sure that McDonalds staff can properly operate their icecream machines by making the machine as easy to use as possible. If you have untrained staff that keep running the machine out of specs because the machine is cryptic and user hostile to them then operator errors could slip past the machine self checks because machines aren't perfectly reliable.
By instructing McDonalds franchises how to properly operate their machines and by updating the UI of the machines to properly warn of dangerous operating conditions the franchises can achieve greater safety for McDonalds customers.
As it is right now, McDonalds is actively endangering its customers with these machines.
I would say it is quite clearly the opposite. Anything which could introduce safety risks to a customer necessitates a visit from a trained service technician to investigate.
And from McDonalds point of view this makes sense and is ethical. The minute you start letting the franchise people make these repairs, they will inevitably cut corners (and get away with it 99% of the time). However, at McDonalds’ scale that would likely result in dozens of people dying at a minimum and huge brand loss.