For the record,
I used to work at Macdonalds and we cleaned out the soft serve and thick-shake (it is basically the same machine) once a day at night time. It is sterilised properly at night, but during the day we did just 'top it up' because it takes about half an hour to an hour to clean and during the day you can't do without the business for that whole time.

Also not sure if the mix was pasturised or not, although i'd presume it would be. It is stored at 4 degrees, however that is in a cold room not a fridge so the temperature could vary a bit, also the machine is cooled, but may not remain below 4 degrees (when you take the lid off etc). Also I think I read that soft-serve was actually the most common way to consume listeria, although yes it is very rare.
Hope this helps,
Suze