Perhaps because some of them are not immune from the vaccine? In which case they could also be putting people at risk. The unvaccinated person may actually be immune through having had the disease though. It's all a bit silly really. Virtue signalling.
Possibly the most ridiculous use of the ridiculous term "virtue signalling" to try and close down debate I have ever seen 
The whole point of vaccination is that you reduce the level and transmission of a disease by a high percentage. Immunity does the same but is less predictable. Some people seem to lose it quickly, others hang on. I still have antibodies but I wouldn't dream of not taking the opportunity to have the vaccine for that reason. Finding out I had antibodies meant nothing because nobody could tell me if I had actual immunity, I still had to be careful.
An airline might decide that to reduce the chance of transmission on the plane (and whatever the airline say it does get transmitted on planes, Hong Kong publish a list of the seat numbers of cases on planes, it is scary, always in clusters) they will require a vaccine passport unless you are medically exempt. That will reduce transmission. End of. No virtue signalling. That is the science....