[DreamSpinner]

Although there are a number of socially related reasons - there are likely some significant immediate commercial reasons to do so.

If you're in a service related industry - do you want staff coming to work if they are (or recently) suffering from digestive illnesses? How would an outbreak of people getting sick after being at your facility look? It could potentially result in very very bad publicity - to the point of destroying your business.

If you're running a smaller business / facility - do you want an employee coming in and spreading the illness to your other workers in the early stages of being sick (and then later taking time off)? This could result in your business lacking sufficient staff to open if you can't use labor hire staff.

The reason why it might be good for businesses to provide an incentive (like sick pay) not to work when sick is that it reduces the risk of negative outcomes to the business.