Spitting on people is of course a disgusting thing to do under any circumstances however your specific claim was that
There is so far as I can see no proof of people being infected by or dying from being spat at.
once again, you are missing the point.
It doesn’t actually matter whether public facing staff (in this case shop staff, but also police, transport workers and HCPs) are dying in droves after being spat on, or whether they are not being infected at all. What matters is the perception, by the staff member, from the media, that they will be at increased risk if they confront people not wearing masks.
public facing roles encounter drunks, drug users and all sorts of socially non compliant personalities on a daily basis. Non mask wearing is now a huge visible signal that the individual in front of them feels strongly enough to not wear a mask. It is just a small step further to think their behaviour could get more antisocial.
Thinking back to my days behind a till we had some very unsavoury characters as customers. Likewise my time as a waitress. My experience of those individuals informs my opinion that those same people are very likely to adopt a non-mask wearing stance, and are more likely to kick off if politely confronted. As usual, it boils down to simple empathy towards others. And courtesy. With masks as a very obvious visual signal of lack thereof.