As the manager of upscale restaurants for the past 20 years, there is NOTHING more disgusting than someone blowing their nose in the napkin. Do people think that machines pick up the dirty linen? No, it's bus staff, wait staff and managers who have to handle the germs.
We have one couple who consistently leave used Kleenex on the table. I have instructed my staff to wear gloves or place a napkin over the remains and dispose of them.
I understand that sudden coughs and sneezes happen to all of us, and I feel for those who have allergies, but Kleenex is inexpensive, easy to hide and not leave on the table.
It all boils down to manners.