I am a medical epidemiologist and infectious disease specialist and one of my areas of expertise is management of the challenge of norovirus. I’ll apologise now for the length of my response.
In Ireland where I work, we see many outbreaks caused by norovirus. It is too costly to test the bowel motions of everyone to see if they are shedding the virus so we test a sample and go on the clinical picture (symptoms) of the remainder of the cases but experiments show us that people who have norovirus will continue to shed it for many days (and in some for up to two weeks after) after their symptoms subside. You are much less likely to spread norovirus; if you do you have diarrhoea (normally formed stools are much less prone to accidental sailing of hands when cleaning your perineal area at toilet).
All the evidence suggests that within 48 hours of symptoms subsiding, your risk of passing on is very high but this falls rapidly over the next few days. However, after five days of normal stool it is still quite possible to pass on the virus.
Forty-eight hours has been chosen as an arbitrary cut off point. Return after more than 48 hours and your risk of passing it on are present but wit good personal hygiene the risk is acceptable. But within 48 hours the chance of passing it on is high, and the risk is unacceptable.
People returning to work too soon can (say in a hotel or in a hospital) pass illness on to other staff and prolong the outbreak.
Moreover, if the infection is passed on to guests (in a hotel) or to a patient (in a hospital) this makes a difficult legal problem. Hotels and hospitals have a “duty-of-care” to their patients/guests and failing to ensure that the risk of norovirus to clients is low then the owner of the hotel/hospital can be legally liable since they put the client in harm's way. Statutory facilities such as hospitals pay their staff for being ill, but many private sector facilities such as hotel staff do not and if ill they have to forgo pay. It is only natural that they will want to get back to work as soon as possible and often overlook telling their supervisor that they are within 28 hours of symptoms.
But I have seen at first hand how likely such a course of action is to introduce a second wave of infection, which is a bad investment for the hotel. It would pay for itself many times over that, during a norovirus outbreak in a hotel, that sick days would be paid to ensure people stay off in the manner that all guidance says they should.